This is important when you add your dye! This is equally important which dye (?) you use. Any physical addition before the polymerization, high probability, will not add up to do the job! You need to find other coloration or tagging molecules which can bind chemically (less efficient than 100% binder will also do) and are retained after polymerization without disturbing/altering the hydrogel properties and appearance. It would be better to have the coloring agent tagged/get bound after the polymerization. Search for your appropriate coloring molecule, I hope you have a clear idea about the coloring/tagging molecules characteristics required for your job!

You can dye with FITC which will react to remaining free amines on lysine residues (assuming your gelMA is not 100.00% functionalised). Works really well, particularly if you can remove the supernatant with the free FITC so only the gel stays brightly coloured.

Have to do it after photo-polymerisation as it interferes with it; won't cure anymore with FITC around.

I think Picrosirius red would work.

Picrosirius red is a good idea. Easier than FITC as it's a straight addition. Both bind to the gelatine which makes it work, as opposed to food dye which is just in the solvent with no preference for the protein and so washes out.

A carmine (a natural dye) displaying a deep bright red color can probably work. A carmine (1-3%) is used as a light absorber in DLP printing to prevent overexposure to light, helping to print a tube-like construct. Please use LAP as an initiator. You may see your red-colored gelMA construct.