To expand on what McD has written..
From the Honeywell RDR-4B handbook, which I think is pretty well fitted to all major types of commercial jets..but which might not be yours!!
Black returns...rainfall rate less than 0.7mm/hr
Green returns.......''............0.7 to 4 mm /hr
Yellow returns......''.............4 to 12 mm /hr
Red returns..........".............Greater than 12mm/hr
Magenta returns.....''...........Not applicable to a rainfall rate.
Quoting from the introduction...
The RDR-4B radar system's turbulence detection capability incorporates a sophisticated Doppler Turbulence Detection circuit that measures the variations in horizontal speed of precipitive particles. If particle horizontal speed variations (indicative of wind shift) exceed the threshold of moderate to heavy turbulence a corresponding display (magenta colour) is provided.
