T-shirt designs

index color separation

Choosing the correct colors, how to…?
Index separation is a process to choose the BEST set of COLOR to represent an image. To reproduce/reprint a digitized image on t-shirt, it is the best if we can print all the colors (like a monitor displaying hundreds, thousands or millions of colors), but this is impossible in screen printing.

Out of the million colors, which is the best set to choose?

The first thing to do is to decide how many color you want to print. This determine by the number of print head available and the cost include. Basically, if there are 8 print heads available : you are allowed to print 7 colors the most, one (the 2nd print head) reserved for flash cure unit. If there are 10 print heads available : you are allowed to print to 9 colors, maybe 8 because there will be circumstances you have to flash cure twice (usually : 2nd print head + 6th/7th print head), very much depend on the print job.

tips : When setting up a new auto/semi auto screen printing machine, DON'T install a print head after flash cure unit, this should be a cooling station. Get advice from the technician incharge, discuss as detail as possible regarding your print job before installation.

After decided the number of color/screen to print, start analyze the image. Look for those majority colors (colors covering most area of the image), without the present of these colors, the image won't look correct. Usually you will have a list of 8 - 12 colors for images like surf designs or illustrations, oil paintings, wildlife art, sports & streetwear t-shirt designs... but if you encounter typical colorful photorealistic image, traditional process color perform better on this. To make a photorealistic image looks correct in Index Separation, we need many colors. [There are disadvantages in traditional process color printing, find out more at Index Color Separation].

Divide the list of 8 - 12 colors into primary and secondary colors.

Primary color	Color that CANNOT be reproduced by mixing any others color together.
Secondary color	Color that CAN be reproduce by mixing 2 other (primary) colors together

For example, in this motorcycle image :
Yellow, Red - Primary colors
Orange - Secondary color

because Orange can be reproduced by mixing Yellow and Red.

Orange = Yellow + Red

You are going to print ONLY the primary colors unless there is extra print head available for secondary color (which is usually not the case for 8 color printer or below).

Don't mislead by the terms primary & secondary
Generally, purple is not a primary color because purple can be reproduced by mixing Red and Blue. But we still choose purple as a primary in this motorcycle image. The reason is Purple cannot be reproduced by mixing the Red and Blue in this particular image. Therefore, the purple is considered a primary color in this image.

After having the primary and secondary concept clear, let us proceed further in detail...

Blue obviously is a primary color in this motorcycle image, but there are different level of Blue. Which is the correct blue to pick?

Pick the vivid midtone blue.

Don't pick the dark blue (shadow). Black is always present as a "darken agent". Black will mix with the vivid Midtone Blue (real primary color) to produce dark blue (unreal primary = secondary).

The golden rule : Any color can be reproduced with the mixture of 2 other colors is NOT primary color.

Therefore, you shouldn't choose Dark Blue as primary color because it is NOT a "real primary color". Of course you still can print it if you have extra print head.

Article by : John Lean
john@teedesign.com