Mike also mentioned that the paint was applied directly to the aluminum model, creating a highly variable ‘anodized’ look. So in certain light, R2’s panels could appear almost black, whereas other lighting would have sections of the metal appear vibrantly blue. In any case, it’s a good bet that the color is somewhat on the reddish side of blue under even lighting.
I’ve noticed that in the slightly overcast shots filmed in Tunesia the blue panels are more electric blue than cobalt blue like they look the rest of the time.