When you would like to change the colour of the second colour stack an you are already in the first stack you can't double-tab onto the second coulor stack to change it.
E.g. you already write in the coulour black in your document. Then you would like to have the colour blue (but you don't have it on the other colour stacks) then you would like to change the colour immediately and you doubletab onto the second coulour stack but this does not work.