I am just a designer. Graphic Design. I will be using places like teespring, spreadshirt. Also eventually some POD companies when I put up my website/woo commerce store.
I do not do any actual printing and never will. LOL
Places like spreadshirt are probably going to be screen printing your product.
They take customer art and do what is needed to fix it for their process. A majority of their customers do not know about the screen printing process, about color separations, cmyk, spot colors, half tones, and other design considerations, so they try to insulate them from that need to know.
Mostly I imagine they just convert customer RGB artwork to their CMYK needs. They should, (one would hope,) be able to spot when a design isn't going to work on a light or dark shirt, and offer advice, such as color changes, or just charge you extra say for employing a white underbase.
If you don't want more specific understanding of how the process can influence design, and want to concentrate on just your drawings, then the best thing you can be aware of is the color count. The more number of colors in your design the more complicated and costly it will be in the case of screen printing. Somewhere in the more than 6-8 color range, it steps off into photo-realistic territory. If it is screen printing, you might want to start working in cmyk mode in your graphics program so they colors will be closer to what you expect.
This becomes more complicated if you trying to do photo-realistic using screen printing. Then you may have to understand more about color separations, half tones and other things so you can design with that in mind, but it isn't a light garment vs dark garment thing.
But if your goal is to have total control over your art then you must include a study of the whole process. You must know your canvas as well as your ink.
I might be completely wrong about screen printing and they are using direct to garment. In either case they should have design guidelines on their site(s.)