You dont have to use powdered colours, you can also use gel colours. I use them all the time. mixed with vodka, always vodka (its just my prefrence) . As above, dont make it too runny, if its too runny, it will be sloppy and your fondant will go soggy.
and as your painting, make sure you use a good quality brush, one where the hairs wont fall out. Also, when loading your brush with the 'paint' Dont over load it, keep it as dry as possible. Sometimes you may need to do two or 3 coats of paint before you get a really strong colour, but let each coat dry inbetween completey before the next. It takes longer, but this way you dont run risk of your fondant going gooey.