Figure out which row is dark red. Then set that row = [1 1 1].
myColorMap = jet(256);
Let's say that your background is say, oh, around -4.5 and the row for that in myColorMap is, say, 128. Then you verify that by looking at the row 128 and see that it is [.5 0 0] which is indeed a dark red. So then you'd just say
myColorMap(128, :) = [1 1 1]; % Set row 128 to white.
% Then apply the colormap
colormap(myColorMap);
colorbar;
