Why is Hellas shown as white?

I've noticed that many Mars charts show Hellas as white as well as the Mars Previewer II program. Well, I've been watching Mars on and off this apparition since Feb. and I've yet to see Hellas as white color like in the chart and the MP2 program! Most definitely nothing like the color of the SPC!

Why have Hellas been depicted as so white?

Thanks,
Ron B[ee]

I think the idea is clarity, not realism...the white tells you "this is a light-colored area", not "this is what it looks like in the ep".

Perhaps because the Hellas basin is frequently the site of frosts and clouds?

http://sprite.phys.ncku.edu.tw/~astrolab/mirrors/apod_e/ap960203.html

Dave Mitsky

In a similar vein, the map that Bruce recently posted reminded me of another mapping convention that puzzles me: Why is syrtis major sometimes shown in a different color than other dark areas? In the map Bruce posted, for example, it's kind of slate-grey.