This is an application for texture mapping. You can create six surfaces -- one for each face of the cube -- and apply your RGB image(s) as the texture map for each. You just have to make sure you flip around the RGB image data so that you're looking at it from the "right side", when assembled into a cube.
Here's a full example:
cdata = flipdim( imread('peppers.png'), 1 );
cdatar = flipdim( cdata, 2 );
% bottom
surface([-1 1; -1 1], [-1 -1; 1 1], [-1 -1; -1 -1], ...
'FaceColor', 'texturemap', 'CData', cdatar );
% top
surface([-1 1; -1 1], [-1 -1; 1 1], [1 1; 1 1], ...
'FaceColor', 'texturemap', 'CData', cdata );
% font
surface([-1 1; -1 1], [-1 -1; -1 -1], [-1 -1; 1 1], ...
'FaceColor', 'texturemap', 'CData', cdata );
% back
surface([-1 1; -1 1], [1 1; 1 1], [-1 -1; 1 1], ...
'FaceColor', 'texturemap', 'CData', cdatar );
% left
surface([-1 -1; -1 -1], [-1 1; -1 1], [-1 -1; 1 1], ...
'FaceColor', 'texturemap', 'CData', cdatar );
% right
surface([1 1; 1 1], [-1 1; -1 1], [-1 -1; 1 1], ...
'FaceColor', 'texturemap', 'CData', cdata );
view(3);
If you're not sure how it all works, try adding one surface at a time -- it might be more clear that way.
