Build custom colormaps

Buillds any sequence of colormaps based on wrgbcmyk colors

This function can be used to build your own custom colormaps. Imagine if you want to display rainfall distribution map. You want a colormap which ideally brings rainfall in mind, which is not achieved by colormaps such as winter, cool or jet and such. A gradient of white to blue will do the
task, but you might also use a more complex gradient (such as white+blue+red or colors='wbr'). This function can be use to build any colormap using main colors rgbcmyk. In image processing, w (white) can be used as the first color so that in the output, the background (usually with 0 values) appears white. In the example of rainfall map, 'wb' will produce a rainfall density map where the background (if its DN values are 0) will appear as white.

Inputs:
colors: string (char) of color codes, any sequence of rgbcmywk
representing different colors (such as 'b' for blue) is acceptable. If a
gradient of white to blue is needed, colors would be 'wb'; a rainbow of
white+blue+red+green would be 'wbrg'.

Example:
[cmap]=buildcmap('wygbr');
%try the output cmap:
im=imread('cameraman.tif');
imshow(im), colorbar
colormap(cmap) %will use the output colormap