Here is an example:
hSurf = surf(peaks);
hCbar = colorbar;
set(hCbar,'YTick',[-5 0.3 6],'YTickLabel',{'Blue','Green','Red'})
You will have to determine the y-values that correspond to the labels you want. For this, you should understand the different methods of colormapping ( direct vs. scaled ) and the axes CLim property. Here are links to some of the relevant documentation to help you get started:
