Ok, try this one on for size. Say for example you’ve done a nice figure illustration but the background isn’t quite the colour you wanted. The problem is that if you start layering your premixed colour on top of your background by the time you’ve gone round all the intricacies of the figure other parts will be drying. You’ll end up with a patchy background. To try and cure this, go and buy a fine mist spray bottle, the finer the better (I use a curling spray bottle). Wash it out properly then fill it with water. You can mask off area’s roughly with pieces of card. Spray the area’s you want to with a fine mist, you don’t want it soaking otherwise the water will run. Just enough to a give a good even coating. Now you can start adding your colour, starting with the finer bits then moving out to the wetened area’s. This isn’t going to work in every situation but it may help if you want to darken or change the colour of your background without affecting your foreground.