From this article, it seems they have choice of color in competition if they are above a certain rank.
"Many rikishi are superstitious and they will change the color of their mawashi to change their luck. Sometimes a poor performance will cause them to change colors for the next tournament, or even during a tournament, in an attempt to change their luck for the better.
Rikishi only wear the silk mawashi during competitive bouts either during ranking tournaments or touring displays. During training, a heavy cotton mawashi is worn. For senior rikishi in the top two divisions (the so-called sekitori), this belt is coloured white, and it is worn with one end distinctively looped at the front. Sagari are not worn during training.
Rikishi ranked in the lower divisions wear a black cotton mawashi both for training in and in competition. In competition cotton sagari are inserted into the belt, but these are not stiffened.
Amateur sumo wrestlers are expected to wear a white cotton mawashi without the looping accorded to the senior professional's training garb."
https://en.wikipedia.org/wiki/Mawashi
