Can't say much for Rit dye and silkspan. About 40 years ago I dyed some silkspan deep red (or maroon) and applied it to a Jetco 'Topkick' A/1 (F1H today) towline glider.
A few hours in the sun at Gardner Field, the old Taft (CA) site, and it quickly began to fade to an almost gray color. Never did see the final result, since the model was lost out-of-sight, overhead, on an official flight. Either the D/T failed, or the thermal was outrageous. At any rate, I never saw it again.
With Rit, some have advised adding salt or vinegar to the hot water dye solution. Don't know if that works, as I never tried dyeing silkspan again.....at least not with Rit.