You need to decide on a length of a colormap, like 256. Then you need to say row 1 is [1,0,0] which is red and corresponds to the min value, which I guess is negative. Then you need to say the last row is [1,1,1] which is white and corresponds to the max value. Now the 0 value of your data may fall into some row in between 1 and 256 and you need to figure out what that is. Easy to do. Something like
zeroRow = 256 * (0-minValue)/(maxValue-minValue);
Now you need to have row 1 to zeroRow go from red to white, which you can do with linspace(), then have from zeroRow to 256 go from white to green, again using linspace(). Not too hard so give it a try.
