Converting the grey image to pure blue with the same intensity is actually very easy, just use that image as the blue colour plane, and put 0 everywhere in the red and green planes:
rgbimage = cat(3, zeros(size(greyimage)), zeros(size(greyimage)), greyimage);
Converting the grey image to a colour that is not one of the primary is a bit more difficult. The easiest way is to do the conversion in the HSV colour space first. The grey image becomes the luminance (value), and you fix the hue plane to whatever constant you want. Then convert the HSV image to RGB:
hue = 29/256; %for example, some sort of orange. Has to be double
hsvimage = cat(3, repmat(hue, size(greyimage)), ones(size(greyimage)), im2double(greyimage));
rgbimage = hsv2rgb(hsvimage);
