IIRC doesn't Nitro (not the whole fuel mixture, but just the Nitro ) have an ingredient in it that changes color when it "goes bad"? I believe it turns yellow?

I had an unopened gallon jug of Tower fuel turn yellow...it was exposed to sunlight over a period of several years. (improperly stored while I took some "time off" from flying )
I still have about a pint of it....been using it for cleaning, etc. like John and Minn have mentioned.

There have been numerous discussions on Nitro, and fuels, etc. in the Glow Engines forum...there should be more info available there if anyone were interested in a search...?