I have recently used some dark coloured chalks/petal dusts.
I thought i just used vodka to soak my brushes--that was apparently a tip on cleaning brushes when you use something else, because the brushes are still dark@@@
what do i do to get the brushes clean?? i think i may have ruined my favourite brush 
thanks for any tips and advice,
lolly
Shake off any remaining dusts or chalks. Then gently wash thremain in the ferrulee bristles with soap and water. Rinse them really well with warm water (hot water will cause ferrule and wooden handles to expand). Shake off excess water, and squeeze out excess from ferrule (metal part) down. Blot dry with paper towel. Reshape bristles using your fingers. Then let your brushes airdry. I usually stick them in a piece of foam-bristles up.
Another tip- buy the best brushes you can afford. If you take care of them they will last alot longer and you won't have bristles falling out on your cake.
Hope this helps
Nancy 
I'm going to give my vote to the cornstarch method as well.
As an artist, I learned to clean my oil paint brushes with turpentine, then wash thoroughly with soap and hot water. Obviously, turpentine is not required to clean brushes of petal dusts and gel colors, so I rinse them thoroughly in water, then wash them with dish detergent, several times if necessary, until all color has been removed. Rinse to remove soap residue. My brushes last for years and years.
Dark colors can stain light colored bristles, but once thoroughly cleaned they can be used for any color without contamination.
Note: Even my red sable brushes shed the occasional hair.
I wash mine with a drop of dish washing liquid and water.
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