Open an image, then click on "Scale->set limits to zscale z1/z2".
Open 3 FITS images, corresponding to red, green and blue map. On the "File" menu, select the red image, then click on "Color->red". Then select the green image on the "File" menu, and click on "Color->green". Then select the blue image and click on "Color->blue". Finally click on "color->RGB mode". A vidéo showing how to do is available here