Step 7: Start Photoshop and open the file you saved in the last step. The first window you'll see before your file is loaded into Photoshop is this Warning. It would appear that Painter uses a different RGB gamut (or color space) than Photoshop -- why am I not surprised? Since we're putting our faith in Photoshop, select the option as shown, which could shift the color space a little, but not dramatically that I've ever found. If you're lucky, you won't even notice the difference.
Here is my picture in Photoshop. I'd be hard pressed to spot any differences with the version in Step 6. However, assuming our ultimate goal is print media, there are some changes in store for this guy.... (actually, we can continue to tweak it even if we want to leave it in RGB mode for the web, but I'm assuming you have a hot book contract you're working on). 
Step 8: To prep the picture for printing, we'll want to convert to CYMK mode. From the menu bar, selectImage->Mode->CMYK Color
You'll see a pop-up window that asks if you want to flatten the image before mode change. Click the Don't Flatten button as we want to keep our layers intact (for now).
Whoa! Big difference in colors as you can see in the results on the right. It will vary, depending on how far out of the CMYK color space your original colors are. In my example, the bright cyan hue of the sea has changed radically (since cyan is only part of the RGB color space) and the picture has become muted overall. Time to put Photoshop to work.
Step 9: Let's work over the sea, first. Remember that color selection layer you created originally in Photoshop and hadn't touched since? Time to make use of it -- first, highlight the color selection layer (unless you left the name as Canvas or changed it to something else). Then from the toolbar, select the Magic Wand tool.You'll want to set up the Magic Wand menubar as follows:
Note: Since my sea areas are broken up, I need to uncheck the Contiguous box.
Now that the color selection layer is the active layer, I select the sea with the Magic Wand tool to get that classic "marching ants" effect:
Select the color render layer -- don't forget to do this!
Now we can manipulate the sea however we want -- modify levels, hues, curves, add filters, etc. etc.
Step 10: I'm going to apply a gradient to add a little depth. The way I like to do this is from the Layers palette. Click on the Create New Fill button at the bottom of the Layers palette (circled in red) and select the Gradient option.
OK, the default gradient (in this example Black to transparent) is not exactly what I want. I click on the Gradient box to bring up the Gradient Editor window and play around for awhile with some gradients. 
In the Gradient Editor, here is a combination I thought was pleasing. I click OK to close the Gradient Editor (for good measure, I save my new gradient in the Gradient Presets list) and also click OK to close the Gradient Fill window.