allows you to define whatever colourmap you want. You can tag a white point on the front of e.g. a Parula colourmap of any size you want, e.g.
cmap = [0 0 0; parula(6)];
In order to make sure only NaNs take the white value though you would likely have to do some messing around with caxis. It isn't easy to define a colourmap for many values where you want precisely one to have one specific colour on the colourmap, unless you map it to true RGB, in which case you can do whatever you want.