The masks really come into their own when used with Photoshop’s “Alpha Channels” – a powerful method of applying and manipulating masks by turning them into selection areas. Try this:
Using Alpha Channels
First, you need to create a new document to work on. This document must be exactly the same size as the map mask you intend using. You can find out the pixel dimensions of the mask by choosing “Image Size…” from the “Image” menu. However, there is a much quicker way of generating a new document with exactly the same dimensions as your mask:
1. Open your selected mask, Select All, and Copy the image. Your selected mask is now copied to the Clipboard, so you can close its window.
2. Choose “New” from the “File” menu. In the dialog box which appears, the image dimensions are exactly the same as whatever is on the Clipboard – in this case, your mask – so you don’t need to change the size. In the “Mode” pop-up menu, select “RGB Color.”
3. In the “Channels” floating window (if this is not visible, choose “Show Channels” from the “Windows” menu), create a new channel by clicking on the arrow at top right (Mac), or button top left (PC), and selecting “New Channel…” from the pop-up menu. In the dialog box which appears, name the channel “mask.” Click “OK.”
 
4. Choose “Paste” from the “Edit” menu. Your mask – which was on the Clipboard – is now pasted into the new channel.
5. Make the RGB channel visible by clicking on “RGB” in the Channels window. The document window is empty.
Into this empty document window, you can now paste anything you like, such as a photograph or piece of art – but it’s most likely that you will want to use the corresponding relief map image from the Mountain High Maps disc. To do so:
6. Paste the corresponding relief map into the document window. The TIFF masks are exactly the same dimensions as their corresponding relief maps, so do not move the image once it is pasted into the window, or the mask will not match the relief map.
7. Now load the mask as a selection by choosing “Load Selection” from the “Select” menu. The mask appears on the relief map as a selected area, which can now be colored.
8. Choose “Adjust” > “Hue/Saturation…” from the “Image” menu. Check the “Colorize” box. Adjust the hue, saturation, and lightness of the selected area by moving the appropriate sliders. If you like, you can save the colors you generate. Click “OK.”
You can now paste any or all of the remaining masks for the map into the “mask” channel and then use them to additionally colorize the relief map by repeating the steps above. Try applying the above process to a photographic image instead of a relief map.
 
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