Cant see any advantage with radar. After all it only knows that its beam has bounced back. Not nesesarily why. Unless of course the screen is also used for chart plotting then I would say colour is a big advantage as trying to read contours off the screen is difficult. Much beter if there different colours.
I agree with Haydn, if you are going to use it only as a radar monochrome is fine but if you want a chart plotter as well then colour is (almost) essential and a 10" screen is a lot more use than a 7" one.
There's a thread on this topic about two months ago, started by me before I bought mine. Opinion then was that the colour ones are easier to read, regardless of having chart attachment. The colour ones seem to be the daylight viewable version, whereas the monochrome are not. I bought colour but have yet to install it. If you are thinking of any of the advanced facilities, like MARPA, you don't have a choice.