Photoshop’s channels feature provides a very fast way of applying different colors to the land and ocean areas of a map image.
Giving the ocean floor a different color
1. Open the base version of your map and change it to RGB color mode
2. Apply your chosen color by using one of the methods described earlier. We have used Hue / Saturation with the “Colorize” option checked, and entered values of 45 for Hue, 70 for Saturation, and 0 for Lightness.
3. Open the “Channels” floating palette by selecting “Palettes > Show Channels” from the “Window” menu. Click on the right-pointing arrow at top right of the Channels palette and select “New Channel.”
4. In the dialog box which appears, name the channel “Ocean mask” and click “OK.” The map image window will now go blank, this being an “empty” new channel.
5. Open the corresponding mask for the map you are using, and choose “All” from the “Select” menu.
6. Choose “Copy” from the “Edit” menu, and close the mask window.
7. The base map window showing the blank channel should now be active (if not, just click anywhere on it). Choose “Paste” from the “Edit” menu. This will paste the map mask into the blank channel.
8. Make the colored base map channel visible by clicking on the word “RGB” in the Channels palette (or type Command-0).
9. Choose “Load Selection” from the “Select” menu. In the dialog box which appears, select “Ocean mask” from the “Channels” pop-up menu, and check the “Inverse” box. Click “OK.”
10. Choose “Adjust > Hue/Saturation...” from the “Image” menu. Check the “Colorize” box, and choose a color for the ocean floor by using the slider controls. We used values of –160 for Hue, 30 for Saturation, and +40 for Lightness.
11. Click “OK.” With the settings described above, our map image now looks like this:
 
Giving the ocean floor a flat tint
1. Open the base version of your map image and change it to RGB color mode
2. Apply your chosen color by using one of the methods described earlier.
3. Define a background color: click on the background color swatch in the tool palette, and use the color picker or custom colors option to select a color. This will be the color used for the ocean tint.
4. Open the “Channels” floating palette by selecting “Palettes > Show Channels” from the “Window” menu. Click on the right-pointing arrow at top right of the Channels palette and select “New Channel.”
5. In the dialog box which appears, name the channel “Ocean mask” and click “OK.” The map image window will now go blank, this being an “empty” new channel.
6. Open the corresponding mask for the map you are using, and choose “All” from the “Select” menu.
7. Choose “Copy” from the “Edit” menu, and close the mask window.
8. The base map window showing the blank channel should now be active (if not, just click anywhere on it). Choose “Paste” from the “Edit” menu. This will paste the map mask into the blank channel.
9. Make the colored base map channel visible by clicking on the word “RGB” in the Channels palette (or type Command-0).
10. Choose “Load Selection” from the “Select” menu.
11. Press the Delete key. This will delete the ocean depths part of the image to reveal the background color that you defined in step 3. Your map should now look something like this:
 
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