What makes drosophila eyes red? and is it stable?
There's been a fair amount of work on this subject but a lot of it is old - see for example this 1945 paper and this 1948 paper.  That latter one in particular goes into some good stuff: color in the wildtype is due to red and brown pigments, which the authors separated by using acidic ethanol (after grinding up the flies).  Here's a 1961 paper from JSTOR that goes into a bit more detail on some of the red pigments (drosopterin, and isodrosopterin, and neodrosopterin) and looks like a good reference for some other, more basic work on Dmel eye pigmentation.  More to the point, here's a 2013 minireview on the chemical synthesis of drosopterins, which is useful mainly for referencing a lot of other work, in particular this overly detailed chapter from 1970.  That last one has tons of info about some of these pigment-causing proteins in insects.

The only particular reference to stability I could find was from that chapter, on page 144:


  The simple pterines (group 2) are chemically stable under normal physiological conditions, while the substituted (groups I and 3) and conjugated pterines are unstable. Light catalysed oxydation of the polyhydroxy side chain at C-6 is a normal phenomenon. The hydrogenated pterines are extremely unstable, especially at pH 7. Tetrahydrobiopterin, for instance, is oxidized within seconds in the light, but more slowly in the dark. [sic]