Most people color their IT or IET (if necessary) with a sharpie or pantone marker, but what of fabric dyes ?
I noticed that RIT brand hot water dyes are for non-synthetics only, but would could I use to dye lycra with ?

I dye synthetic wigs with RIT all the time...

I see no reason why it wouldn't at least TINT invisible Thr**d to a darker color..

The radio controlled car guys used to dye the white nylon parts on their cars with RIT but they used a higher concentrate of dye and boiled the stuff for a long time. It might be tricky to do that with IT.