You can use this technique to make painted cut-outs or you can apply the paint directly on the cake.
If you are making cut-outs, roll out the flower paste (gum paste) to the thickness you need. I used wooden lollipop sticks as spacers. Let the rolled out piece to dry from both sides for a few minutes.
You will need a sponge. I used a piece of a dimple foam pad.
Use a brush to apply some paste colour on the rolled out paste. The colours you use will get mixed together so choose colours which look good once combined together.
Don't apply too much colour on. You can add more later if there's need. The more you apply, the longer you will have to wait for it to dry.
The paste colours give a slightly shiny finish. If you want a matt effect, use dust colours dissolved in alcohol (see step 12).
I used Wilton paste colours (Sky Blue and Lemon Yellow).
Use a damp (not wet) sponge to spread the paste. Don't rub the surface of the paste, but tap it gently with the sponge.
Don't apply too much water on the sponge - it will dissolve the flower paste.
Try to spread the colours so that there are areas with different shades. If you are painting a larger area, you might need to wash the sponge from time to time to remove the excess of the paint off.
Add some colour if there's need.
Once you are happy with how the colours look like leave the paste to dry for some time - until the surface is no longer sticky.
I used a blow dryer to quicken the drying ;)
Ok, so I guess it's a good idea to use gloves for this ;)
When the surface is no longer wet you can sprinkle the surface with corn flour - that way the paste will be less shiny.
Remove the excess of the corn flour with a dry brush.
You can add some gold or silver colour here and there. Tap the brush against the surface of the paste.
I used gold Rolkem highlighter dissolved in a small amount of alcohol.
Cut out the shapes :)
As I mentioned earlier, you can use dust colours dissolved in alcohol instead of paste colours.
If you use dust colours you get a more delicate and matt effect.
The paint shouldn't be too thick or too thin. I suggest testing the consistency on a spare piece of paste first.
You can use a toothbrush or a hard brush to splatter f.ex. gold or silver colour on the paste. The toothbrush of course needs to be clean and used for cake decorating purposes only ;)
You can stick the painted cut-outs randomly on the sides of the cake or you can create geometric patterns. You can find a lot of beautiful geometric pattern inspirations in the Internet - I usually use Pinterest or Google Images to look for ideas.
If the cut-outs dry out too much (and you want them to curve around the cake) - steam them slightly (from the back; if you steam the painted side it will become sticky).
If you are using this technique to paint a whole cake, I suggest leaving the cake to dry for some time after icing it (and before applying the paint), so that the sugarpaste gets more firm - you don't want to indent the cake when applying the paint ;)