So, after everything we've discussed in the first part, there is still one big question left unanswered.
Should I design in CMYK or RGB and which colour space should the final output file be saved in?
During the designing process, I always work in CMYK. It excludes any unprintable colours from the palette and gives me a better idea about how the final product would look.
I've read some people saying that their printers actually have a greater gamut than CMYK SWOP (Adobe's default CMYK space), so they use RGB and while there will be some colour loss when comparing the final product to the RGB file, it still gives them a greater colour range. I don't know much about the various printers' capabilities, so I won't call bullshit on that one, but the CMYK SWOP space is big enough for me in any case, so I stick to CMYK, and I suggest you do too. (If you had a super-duper printer with a wiiide colour gamut, you probably wouldn't be reading this blog entry anyway.)
Next, you need to know what method would be used when printing the t-shirt. If you are doing the printing yourself, you should already know. If not, simply ask the people from the printing service or consult the FAQ of your chosen online retailer. The options are basically two:
1. DTG (Direct To Garment, or simply digital printing). In this case, you should save your final file in RGB. Inkjet printers would automatically convert the colour space to RGB, but you don't want to trust a printer with that, so it's best to save the file in RGB by yourself.
In addition, the best format to save in (unless the printing service accepts vector files) is png (it is lossless, supports transparency and doesn't take up much space), and png doesn't support CMYK, so you will be forced to save in RGB anyway.
2. Screen printing. With screen printing, you should not only use CMYK, but probably spot colours, if the press is manual. With automatic presses four colour printing is OK too. In addition, screen printing would require image preparation, which can be done by you (maybe I'll do a blog entry about colour separation in the future), or by the printing service. Again, you should ask them for guidelines.
Coming up: Graphic Design Basics: Colours Pt. 2 - Rich/True Black vs Default CMYK Black.