The answer to your question depends on the specific weather radar product. Some weather radars have a 4-color precipitation mode, where magenta will show the heaviest rainfall (for example, one manufacturer's 4-color radars show magenta returns for "areas of very heavy rainfall rates of two inches per hour or greater.")
However, you're asking about weather radar that has a turbulence mode, and uses magenta to show areas of suspected turbulence, right?
The difference is that many radars (see first sentence in para 1) use wind data to provide the magenta turbulence display, in addition to their water reflectivity-based radar which provides the green, yellow, and red displays.
Significant changes in wind velocity are a very good indicator of potentially severe turbulence, so that's what the radar is looking for in turbulence mode. Often these magenta displays are found in the midst of the red returns -- no surprise there. The red is telling you about the heavy rainfall, and the magenta display is telling you about which specific areas have the worst turbulence potential, based on the winds.
When is this useful? You are correct when you state that red returns are probably a good indicator of turbulence anyway, and yes, you should most times avoid the red.
But, there are times, though, when flying in tropical areas, at fairly low altitudes (such as on departure or arrival) when you're only flying in steady rain (no turbulence at all) but you'll still get red returns due to the size of the water droplets and the amount of rain falling. In this case, the turbulence mode magenta returns can be helpful because they can help you differentiate between the red returns you really need to avoid, from the red returns simply caused by large rain droplets.