hi there..
what type of color do u use?
since i use acrylics, i used this method which i accidently found.....
i just soak my model in concentrated laudry liquid detergent with water solution....in malaysia, i use the brand Kuat Harimau (means Powerfull Tiger)

no specific ratio..i just pour the desired quantity, mix it with water....and the just put the model into it..
for newly painted models, the paint will be removed in less then 30 mins...without further rubbing the model..
for models that have been painted for a while (means the color particel have been fully harded) then, it may took longer time...maybe overnight and u may have to rubb it with a soft cloth ....
this method work best on me so far...the detergent is powerfull enough to remove my tamiya and gunze paints...
i accidently found the method when i'm trying to clean my model from dust...after wipe it with brush..i thought th model will need a nice warm bath....to my horror, the paint job is removed immedietly when i run the first flow of detergent and water mix thru the model....
good luck