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Sounds like you are getting close.
Here is a sequence for painting 250/250/250 white onto your image. There are many other ways to do it and ways to create the mask other than painting it on but it is one of the simpler ways and agrees more or less with what Chuck was trying to tell you.
1. Load your dog pic. I am showing it as a normal layer here but it could also be a background layer, at least starting out.
2. Create a new solid color layer by clicking on the Create new layer icon...
3. ... and select Solid Color.
4. The color selection windows will come up and you can specify the color - in this case 250/250/250.
5. Now click on the mask icon of the new solid color layer. It should show white brackets around it if it is active.
6. Now do a Control I to invert the mask. For masks, white reveals and black conceals.
7. Now with the solid color mask still selected, use a soft brush with pure white (255/255/255) foreground color and paint on the areas of the image you want to be your forced 250/250/250 white. It will look like you are painting 250/250/250 white onto your image but what you are really doing is painting pure white on the mask which reveals the 250/250/250 solid color layer.
8. As I mentioned before, it can be handy to see the actual mask full screen rather than the masked image in order to fill holes in the mask. To see the mask in the main window, Alt-click on the mask icon. To go back to the image, just click on the image icon.
Some other comments:
- You don't have to use a solid color layer. You can fill a new, transparent layer with a color also as you apparently did in one of your attempts.
- Layers can have both bitmap and vector masks. Bitmap masks are usually more useful for this type of compositing.
- If you use filled layers instead of solid color layers, you can use the eraser tool directly on the layer rather than using a mask. Normally, however, masks give you more flexibility and undoability.
- You can mask in or mask out - whatever you find best for a given image. It just depends on the order of the layers and your work preference.
- You mentioned that you weren't seeing solid 250/250/250 in your attempt. There can be several reasons why: You left some holes, you are seeing the soft edge of the brush you used, you were using the brush at less than 100% opacity, the color layer is set at less than 100% opacity, etc..
- For my simple example above, you could also just paint 250/250/250 on a transparent layer above the image layer but learning how masking works will help you for other things.
Hope that helps.