Do this:
% Colormap "where the value of the 0.2 to 1 is red and
% -0.2 to -1 is blue, and the rest is green.
numColors = 256;
vec = linspace(-1, 1, numColors);
cmap = zeros(numColors, 3);
% Make everything green to start with
cmap(:, 2) = 1;
% Make red from where vec goes from 0.2 to 1.
redIndexes = vec >= 0.2 & vec <= 1;
cmap(redIndexes, 1) = 1;
cmap(redIndexes, 2) = 0;
cmap(redIndexes, 3) = 0;
% Make blue from where vec goes from -1 to -0.2.
blueIndexes = vec >= -1 & vec <= -0.2;
cmap(blueIndexes, 1) = 0;
cmap(blueIndexes, 2) = 0;
cmap(blueIndexes, 3) = 1;
cmap % Echo to command window so we can see it.
