obviously the button on the craft bench is gettinh hot and change his color. at the moment it change is color the more items re craftet in a row.
i interpreted in that way that the craft button symbolizes an anvil(the spark sprinkling).
also some items need more hammertimes respectively craft actions.(iron ingot 1 time / rotor 3 times)
my suggestion is that the coler cahnge per craft actions/hammertime and not per item that is craftet. it would be way more logical in my opinion.