The problem is that when loading another PNG (most likely gif as well) the color reduction slider is fixed onto value field not the % of current slider position.
If we set slider to 100% and load 13 color image it will put in the field 13, slider is still on 100%. Then we load image with 40 colors, but the field stays on 13 and slider is rescaled, so we end up with color reduction even if we had previously slider on 100%.
User fix: none, suggestion: RIOT should re-read value from slider not from the field. (or make it as a setting to decide for user)
If doesn't seem to be related to 2/4/16/256 color depths.
This is very nasty when we use maximum compression, because we have to wait for a recompression twice. First after the load, then after we correct the slider to 100%.