The hue slider does not contain all colors - if a stamp is originally red, you cannot turn it fully yellow/green, and vice versa. Blue, turquoise und purple are always possible.

The BABY DRAGON is a good example - you cannot turn it fully red, even with the saturation and contrast to max, it only gets orange-and-pink.

It would be nice if the colors would change more like on a rainbow.

Alternatively, you could offer every stamp that is red, yellow or green in variations. Please offer a dark red dragon baby!