[edit: Fix spelling errors in my remarks. No changes to the code.]
x=0:.01:6; y=0:.01:2;
[X,Y]=meshgrid(x,y);
Z=sin(pi*X/2).*sin(pi*Y/2);
% plot results
figure
surf(X,Y,Z,EdgeColor="none");
axis equal; grid on; colorbar
xlabel('X'); ylabel('Y'); zlabel('Z')
%make custom color map
hue=[0:.01:.67,.66:-.01:0]';
nc=length(hue);
colors=hsv2rgb([hue,ones(nc,2)]);
colormap(colors)
The key is to recognize that progression from red through green to blue corresponds to hue=0 (red), hue=0.333 (green), hue=0.667 (blue). Then you want to go backwards, from blue to green to red. Therefore you can create a hue vector which goes in steps from 0 to 0.67, then back to 0. Then convert the hue vector to RGB colors, since the colormap() function wants a set of RGB colors. When converting the hue vector to RGB colors with hsv2rgb(), use a column of ones for the saturations and a column of ones for the values (S and V in "HSV").
