I have (in one of my Mechanical eng. -books) color codes for heated steel (from red, orange, etc.) depends of the temperature. Probably somewhere (Mechanical engineering/Materials books) exists list like that for the Tantalum, and from the colour of the heated Ta you can guess (optimal) working temperature / anode dissipation, if I`m correct? However, try your college connections:-). I can (tommorow) ask my friend, he`s the assistent on the College of the engineering (dr.sci), specialist for the metals.