How many balls?

To find the probability of the two balls being different colours, we can work out the probability of the two balls being the same colour and subtract this from 1.

Probability of two reds = $\frac{3}{12}$ x $\frac{2}{11}$

Probability of two whites= $\frac{4}{12}$ x $\frac{3}{11}$

Probability of two blues = $\frac{5}{12}$ x $\frac{4}{11}$

So the answer is:

1-(( $\frac{3}{12}$ x $\frac{2}{11}$ ) +( $\frac{4}{12}$ x $\frac{3}{11}$ ) + ( $\frac{5}{12}$ x $\frac{4}{11}$ )) = $\boxed{0.71}$

1 - (3/12)(2/11)-(4/12)(3/11)-(5/12)(4/11)