You can define your own colormap as follows:
x = randi([0 3],10,10);
colors = [0 0 1;... % blue for 0
0 1 0;... % green for 1
1 1 1;... % white for 2
0.65 0.16 0.16]; % brown for 3, etc.
figure;
imagesc(x);
colormap(colors);
