What is a Colorant?
A colorant is a substance used to color a material, such as thermochromic and photochromic dyes aqs well as pigments in paint.
What does thermochromic mean?
thermochromic = thermo + chromic
thermo–relates to heat
chromic–color changing
thermochromic materials are colorants that change color in response to changes in temperature.
What are HAT Stickers?
HAT Stickers aka Heat Activated Thermochromic Stickers
Heat activated thermochromic dyes have a specific color at room temperature. The shape of the thermochromic dye molecules absorb and/or reflect specific colors of visible light at room temperature. The thermochromic dye used to make the HAT mixture shown in the chart, reflects blue light when under white light. (White light contains all the colors of visible light).
Each row of the chart shows the color of thermochromic dye, the paint pigment and the color of their mixture at a spec
ific temperture.
Row 1 Room Temperature: At room temperature, which is about room temperature of about 20 to 25 0C (68 to 77 0F), the molecules of the thermochromic dye reflect blue light and the molecules of paint pigment reflect yellow light. When combined, the mixture contains molecules of dye and molecules of paint pigment. The dye and paint molecules are relatively uniformly distributed throughout the HAT mixture. This means the HAT mixture is not a new substance, instead it contains separate particles (molecules) of thermochromic dye and paint pigments). At room temperature the thermochromic dye molecules are reflecting blue light to your eyes, and the yellow paint pigments are reflecting yellow light to your eyes. When reflected light waves entering your eyes are close together, what you see is a perception of the two light colors blended together. Thus, the color of the HAT mixture is the blending of blue + yellow, which is green.
Row 2 Activation Temperature: At the activation temperature of the thermochromic dye, which is 27 0C (81 0F), the thermochromic dye is transparent, meaning light passes through the thermochromic dye molecules, much like light passes through a glass window. The yellow paint pigment is not affected by the higher temperture, thus it continues to reflect yellow light. Thus, the HAT mixture appears yellow at the activation temperature of the blue thermochromic dye.
Row 3 Room Temperature: The molecules of dye and pigment are allowed to cool back to room temperature (aka ambient temperature). Thus, the thermochromic dye molecules reverse back to their original structure and again reflect blue light. As before, at room temperature the dye and pigment mixture reflect blue and yellow light which your eyes perceive as green.
Troubling Shooting
I prepared an activity using HAT stickers for an elementary after school program. I explained that the green stickers would change color if rubbed. GREEN STICKERS? 
There were no green stickers. Instead, all the stickers were yellow. The problem was that the room temperature was at or higher than the thermochromic dye’s activation temperature of 27 0 C (81 0 F). I certainly didn’t doubt it because sweat was dripping from my face. Thankfully there was a refrigerator in the building, but I discovered that there was no ice cubes in the freezer. With permission, I took frozen food packages from the freezer so kids could cool their fingers and touch the HAT stickers. They were excited to see the stickers change colors. Their cooled fingers left green prints on the yellow stickers. But, because of the hot room temperature, their green finger prints quickly turned yellow.
HAT STICKER Craft
I tore the HAT stickers in half and wrapped each piece around large paperclips. The kids were then able to thread a ribbon through the paperclip and have their very own color changing necklace. HAT Stickers can be purchased here JVC HAT Stickers For Fun Science
