This is a standard polymer infused with a colour-changing additive. The coloured additive becomes transparent at a given temperature, revealing the polymer’s base colour. The base colour, or "hot colour", can be any colour as long it is a lighter colour than the additive.
Thermochromic plastic is used for lids for coffee cups, that indicate whether the coffee is hot or not. The sample is spoon for babies that changes colour if the food is too hot for a baby's mouth.
The additive can be compounded into a range of different plastics including polyethylene (PE), polypropylene (PP), and polystyrene (PS). It is available with colour-change temperatures ranging from 5ºC to 41ºC. The additive is damaged at high temperatures, though; the maximum moulding temperature is limited to 235°C. It is available in 11 standard colours including green, blue, orange, yellow, brown, magenta, and pink.
You get one spoon for baby food that changes colour from orange to yellow as seen on the picture.