The overwhelming majority of printing on fabric, such as a t-shirts, hoodies, aprons, etc. is performed using what is commonly referred to as Spot Color, 4-Color Process, or Indexed Color. The exception to traditional screen printing is called DTG (Direct To Garment). See What is DTG? article for more information on this printing process.
Spot Color: The term Spot Color actually describes the process itself. This is where each individual color in a design is separated out and printed separately. Please see the What is Screen Printing & How Does It Work? article for how this artwork is taken from a file and Separated then transferred to Film and eventually made into a Screen used for printing.
The advantage of using Spot Color printing is that it is pretty easy to print as the colors are distinctly separated in the design and can easily be printed individually. In the Figure 1, to the left, the design was printed on a white jersey so we can easily separate the design into three (3) distinct colors — Gold, Navy, and Gray.
This is the simplest form of screen printing and screening skills that every Printer learns from day one, though not everyone does it as well as others. There is a skill required to align the three color screens to get a good clean print. See the What is Screen Printing & How Does It Work? article for more on this.
But like everything else in life, it is not as always as straight forward as just printing solid colors. For instance, if you printed the design in Figure 2 on a piece of paper it would not be a problem. Your Epson or HP would print all the gradient colors without a hitch. However, when printing on fabric this would be a problem as printing Spot Colors mean that you print a distinct SOLID color. In Figure 2 it would be either purple or pink, along with white.
On the surface, in the Spot Colors world each shade of the gradient is a distinct and different color from every other color in the gradient. So in the below design, we could potentially have dozens of colors; and since each color to be printed in a project increases the price, it would be EXTREMELY expensive to print all the gradient colors separately.To get around this we do what is called using Halftone Dots to simulate the gradient.
Halftone Dots: A halftone is simply a group of large and small dots that when viewed at a distance, have the appearance of continuous shades of gray or color in an image. Below is an example of what I mean.
On the Left side of Figure 2 is a closeup of the Halftone dots as they would be printed on film to make printing screens. The dots have been zoomed in quite a bit to show the individual dots.
On the right side of Figure 2 is how the human eye would see the dots from a sufficient distance.
As you can see, the same number of dots are used for the top & bottom of the picture, it is just the dots become smaller the higher you go in the picture giving the impression of a dark to light gradient.
When multiple colors are involved it becomes somewhat more difficult to accomplish. For instance, in Figure 2 we would need to use Halftone Dots for both the purple and pink colors to make them lighter until they run up against each other and then we use some of both the purple and pink Halftone Dots next to each other to give the impression that they are blending together. Depending upon how extensive the gradient is we can sometimes take just two colors and make the entire design, as is the case with the design in Figure 1. However, sometimes we need to add another color such as if the design was going from Black to White we might add Gray Halftone Dots in between the Black and White. So determining the number of colors, when gradients are involved, is not a simple task and normally requires us to look at the artwork to make a determination.
But what about when you have a LOT of colors that are blended together? In this case it would be extremely difficult to create all the necessary Halftone Dots and would still probably end up with a lot of colors and a lot of cost. This is where Process Color comes into play.
4-Color Process: In some cases there are so many colors in the design that it would be cost prohibitive to print it, such as a full color picture. So what we want to do at that point is to emulate a full color picture with the least number of colors possible. One way to do this is by using DTG. However, when printing a large quantity of t-shirts or on apparel other than t-shirts or if you want the print to last longer, then DTG is not the answer and we have to use Process Color printing.
This is where we use special ink in four (4) basic colors, CMYK; which stands for Cyan, Magenta, Yellow, and Key (Black). These colors make up a “subtractive color model”; which is to say that the higher percentage of cyan, magenta, and yellow you smear onto a white sheet of paper or fabric, the less amount of light that will reflect through, ultimately creating a black color. You can see this in Figure 4 when the Cyan, Magenta, and Yellow all meet you end up with Black. From these colors we are able to create all sorts of other colors. Or at least enough colors to print a full color design.
In Figure 5, to the left, is the actual digital image provided as the example to print the t-shirt. In Figure 6, to the right, is the actual printed white t-shirt using 4-Color Process. It is a pretty good representation, however when we print the inks to get all the colors to display some of the colors were not as bright as the original image to the left.
And as I eluded to above, there are drawbacks to 4-Color Process (there’s always a trade off). One of these is that it is much more difficult to control how exactly the print will look when actually printed, as shown in Figure 6 above. With Spot Color it is pretty easy. We just group all of the same colors together so all the blues are together, all the yellows, etc. However, with 4-Color Process we are blending a percentage of one of the 4 colors together with the other colors to try and make a totally new color. The issue is that when we use a specific percentage of say Magenta blended with a specific percentage of Yellow to make Orange — that same percentage of Magenta may not work with Cyan to give us the Purple color we want. So we have less control over how the color turn out. Additionally, because the whole concept of 4-Color Process is to block out percentages of white to give us lighter or darker colors — if we do 4-Color Process on anything other than white apparel the colors in the design can be greatly impacted by the color of the fabric. Trying print the same design on a white piece of paper and then on a yellow or blue piece of paper to see what I mean. This requirement is very similar to DTG. While 4-Color Process can emulate a full color design, it does NOT give you the higher definition quality print of Spot Color.
So now we see how Spot Color printing works and even how we can get a gradient. But what if you have dark colored apparel with a lot of blended colors. We have already said 4-Color Process won’t work. Or you just have a lot of colors in the design (even on white apparel) that you want to reduce to a smaller number of colors. This is where Indexed Color can sometimes work.
Indexed Color: Index color is a process where a design with lots of colors is reduced down to a limited number of colors. This process uses dots, similar to what you would do to create a gradient, but instead of using halftone dots, Indexing uses the same size dots but converts the design to a diffusion dither random square dot pixel pattern – in laymen terms it gives you a more moiré (or watery / wavelike) appearance vs Halftone Dots which are elliptical in nature.
This wavelike pattern allows us to get a more cleaner print and provides a brighter print with higher contrast. And as I mentioned, you can print this on dark colors as well as light. In Figure 7, to the right, you can see an image that was Indexed using 6 colors. This is the actual black t-shirt with a full color print on it using Indexed Colors.
Now here comes the bummer side to Indexed Color. Normally to Index requires 6 to 8 or more colors to get the image to look good. This is a lot, but it is much better than 20 or 25 and is about the only way to get something like emulated full color on darker colored apparel.