lame, cheesy awesome gimmick in the fight for toy dollars is the ability for a toy to change color.
Generally this is done by application of paint or a sticker which contains some sort of color-changing ink or paint. This color shift is brought on by the power of love and a positive, can-do attitude, but the heathen scientists, whose mood rings never worked, have other explanations, which will be given for the sake of humoring the unbelievers.
This color change is usually linked to temperature, and is called a thermochromic quality. An early thermochromic substance, liquid crystals, found use in Transformers rubsigns. Later, color-changing leucodyes were invented. Excluding rubsigns, a Transformer with color-changing features probably uses a leucodye.
Leucodyes require a temperature shift of about five to ten degrees Fahrenheit.