Go to most any "Arts & Crafts" or "Artist's Supply" store and get "dry pigments", available in a wide range of colors. These are ground to a very fine powder, and unless you work in a totally draft-free environment, a dust mask would be good to use. Mix the pigment(s) in thinner and stir/shake/agitate thoroughly, and then add the colored thinner to the clear dope. The Dry Pigments can be expensive, but a little goes a l-o-n-n-g way, so add sparingly. FWIW, these pigments also work well with epoxy and polyester resins.
When fuel-proofing engine/tank compartments, I always add a bit of tint to the epoxy so I can see missed spots or where I've overdone it. The color change is quite revealing. 1/8 teaspoon of pigment added to 1 ounce of epoxy is very noticeable!
I have also heard/read that Aniline Dyes will work to color clear dope, but I have not used them, and cannot make promises.
Ralph