Hi Sonya
Welcome to the TSF!, and thanks for your patience awaiting a response.
There is a transparent waterbased ink from Matsui Color with dye-like qualities on white shirts they call the RC301 Series.
It is available in Pantone
TM Primary Colors that are mixable either using that guide book or mixed intuitively.
You can increase the transparency by adding more Clear 301C to the blends for secondary color overlapping or fading tone watermark effects, without effecting ink performance.
Another option used by several art students we supply at ASU are the RC301 Series Process Colors (Cyan,Yellow, Magenta).
Mixing is then even more simplified, basically following primary color theory, and can also be reduced down further in opacity with the Clear Base.
Note that this ink is formulated for heat-curing to acheive the best after-market wash-fastness and is not an air-dry formula.
Happy trails!