Unless I misunderstand the question, an .ai file is fine. I am assuming you want to print digital transfers from an inkjet printer with pigment ink. You would print from Illustrator onto your transfer paper. What is more important is a correct icc color profile made for your specific printer, ink and media. Unless you have an icc color profile, your colors may or may not be very accurate. The vendor you buy your supplies from may have color profiles for the products they sell. A calibrated monitor helps in the equation as well.
I designed everything in RGB. My new printer, however is a
6 color printer.
Color profiles are a bit foreign to me. I do however understand off set printing, cmyk, spot colors, etc..
from my days working at a print shop. I do not understand
how all that translates to my 6 color ink jet.
Your 6 colour printer most likely separates from RGB to its own colours via ICC profile. If you send it CMYK data, it'll translate that first to RGB, and then generate the separations with its own 6 colour system.
However, not all RGB colour spaces are the same. The ICC "PCS" or profile connection space tends to use a different set of values for its component colours than those used by most RGB profiles, which are based on the colours that screens render easily. The sRGB colour space is horrible, really washed out, being a worst-case-scenario estimate of what a screen can do.
Best file format to save in is this case is an RGB illustrator PDF with an Adobe RGB profile.
You could get into calibrating your screen, and embedding the profiles for your printer into your documents, but I've found that by and large Adobe RGB gets better results. Every time you narrow the colour space, you lose quality.
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