Getting the right color with reef photos isn't easy. I struggle with it myself. If you use the expodisc to set your white balance you will have an 18% gray image to work with in photoshop. I'm not sure about your version but you should be able to open the subject photo and the image you made as your WB shot, go to the image pull down menu, then adjustments, then curves. you should now have both photos open and the menu for curves. Click on the subject photo to highlight it, now click the center eyedropper in the curves window, this is used to set the gray point. now place the eyedropper over the expodisc image and click on it. this should change the color blalance in your subject image. If the expodisc image got a good exposure your subject image should be closer to the right color rendition!