Just made some measurements - due to noise made by some of participants of the taboo forum. Really, noise helps... Styrofoam cups have non-linear response to different sources of light. The colour temperature measured for different kinds of light sources with and without Styrofoam cap can deviate as much as 600K from direct measurement; typical value for daylight is 270K. Equipment used: Gretag Spectrolino spectrophotometer to see resulting spectral power distribution; Gretag EyeOne colormeter; Minolta Colormeter IIIF. Miscellaneous equipment - Sun, clouds, different lamps, Nikon and studio flash, colour compensating filters from Lee Filters.