I can help with this one. RGB is for computer images, CMYK are for printed. The pixels on computer monitors use red green and blue to make every color, green gives a better color range than yellow, which is why it isn't RYB, so when you're doing something that will be used for web graphics, video games, etc. you use RGB. I've noticed at times when I didn't switch from CMYK and saved my GIF and JPEGs they wouldn't show up when I put them on the web. If you're making a magazine, sign, poster, etc. you get a better range of color with the four color system. Now as far as screen printing..... it doesn't make a difference if all you're doing is printing in black to make the transparencies, but if you're doing heat transfers, DTG or dye sublimation it would be best to use CMYK because it breaks everything down into those four colors and comes out more accurate than if you use a file based off of three colors that translates as best it can when it goes to the printer. I seen someone say CMYK looses color or something like that, in these cases you might want to calibrate your printer because that's usually what makes it print differently than what you see on the screen. Lap top and other flat screens sometimes make it hard to guage a color because it can look one way from your current perspective, but if you stand up and step off to the side, the whole screen will look darker or brighter depending on where you're standing, so that aspect alone can throw you off in terms of the actual shade of a color and how it looks when it's printed. I hope this helps
