These two plates from the same edition of the Botanical Magazine (edited by William Curtis) are an illustration of what can happen over time to these fragile prints, and possibly to the effect of different paints used by the colorists.
The plate to the left is a good representation of the actual color of Sinningia cooperi. The plate to the right is showing the effects of oxidization on the red paint, with patches turning a dark reddish brown. The overall color is deeper than on the other plate, which could be a function of oxidization or perhaps the use of a different paint by the colorist.
Deterioration of these old plates will happen much more quickly under some conditions. If kept dry and dark, and preferably cool and away from damaging insects, they can last in pristine condition for a very long time. In humid and warm environments both the paper and the paints can deteriorate, and if exposed to light they will fade quite quickly.