This is an example of how colors can come in from customers already converted to a CMYK file and the differences of color expectations. Some customers may have heard or experienced that all printing is done in cmyk. Paper printers using other print methods may print in cmyk and many digital printers for posters, signs, banners etc, print in cmyk or have additional bump colors like Light Cyan, Light Magenta to beef up color. So they may in fact print cmyk, but the file that they provide (for the printing), should still be supplied in RGB. This is because RGB is more raw, and when the printer gets that file, they will do their own color conversion from RGB to CMYK based on their color capabilities for that shop.
For apparel printers, most all jobs are not printed in CMYK, nor are they printed in RGB colors. We can, and do print in cmyk, but only in specific situations where needed. It’s far more efficient and consistent to print most art as spot color or simulated process colors. When we get art files in, we do (or should) prefer the RGB files over CMYK files. We can still use the CMYK files, but with the understanding that colors have already been changed or stripped out of the file. When art is sent in (as a CMYK), those colors
and the original creators color profiles are embedded or locked into the file. This then, presents a problem when trying to pick colors to match what was to be the original. Color profile settings can be drastically different from machine to machine based end preferences for your shop that you have put into your own computers CMYK profiles.
When the art is created, You may like the colors on your screen but that same file and colors are seen on the next computer differently (based on the next computers own CMYK color profile settings. In other words, it’s not going to look like the same colors as the previous person had seen. For the most part, they will be pretty close, but some files, some elements with certain colors can be drastically different. In addition, once you convert to cmyk from rgb, the colors are dumbed down to a much smaller color gamut and many cannot be produced in CMYK. Even the Pantone book does not contain colors that you may see on screen. In some cases, the color differences are drastic such as with deep purples and deep blues. The best way to send files is to keep them in RGB and even then, when we pick a Pantone color to match what is seen on screen, it will still only be close, but closer than if I were looking at your CMYK conversion.
Art was created by Don Howard,
Donhoward.org used in this video for screen print educational purposes. Printed by IDP Huntsville Al. Separated by Dot-Tone-Dan Campbell Dot-Tone-Designs VIP’s
https://youtu.be/ysmwpGJiZPs 
