mymap = gray; %copy the grayscale map
mymap(SomeIndex,:) = [1 0 0]; %set one particular entry to be red
imagesc(YourImage);
colormap(mymap);
This code does require that you figure out what map index the value X will be mapped to by imagesc.
There are alternatives involving overlaying a red image of exactly the same size that is transparent everywhere except for the places X is. This requires OpenGL (no other renderer handles transparency)
