This seems so basic, I’m almost ashamed to post it here… almost
But, in case you're interested, here's a small tutorial on how I get a nice artificial watercolor-look in Photoshop 7. I'm sure that Painter actually has nice watercolor brushes that render all this extra work obsolete, but Photoshop users might find it interesting. It's pretty simple, too.
Step 1: Put your linework on a transparent background. There are a number of ways to do this, so I’ll just briefly say this. Clean up your linework first (adjust contrast, etc), then:
1. Click the 'channels' tab.
2. Click 'load channel as selection'.
3. Click 'delete' to get rid of the white.
4. Select 'inverse' to select your lines.
5. Hit ctrl C (to copy your selection) and ctrl V (to paste into a new layer).
6. You're done!
The reason I like to do this is it allows you to change your inkwork from black to... whatever color you like. Brown lines look especially nice if you're looking for a real old school traditional feel. Make sure you lock the transparency of the layer before beginning work on it. To change any linework, simply unlock and alter it as you normally would. Here's the linework for my piece. I've got a white background behind it so you can see it. I've already changed some of the line colors:
Step 2: Blend colors. So, to get the nice gradations in color that occur when you're using wet-on-wet watercolors, you can use the healing brush, the little 'band-aid' on your tool pallette. The way I do this is pick a portion of the painting and block in all of the basic colors. Then go back and blend the edges with the healing tool set on 'multiply' (on your tool settings). Here's some random scribbling so you can see what it might look like:
I like to work on each 'element' separately, that way I can't screw up the whole thing at once. It's best to just go crazy with color, then use the healing tool, then clean up around your lines with the eraser. Here's the monster, cleaned-up:
and the background - all done the same way - on a separate layer. It gets a little boring after a while :? :
Step 3: Time to add texture. Once you've got your finished image all sorted out - colors adjusted, etc. - you're ready to add some texture. Save your finished pic as a jpg or flatten your image (you could do this in your PS file, but it gets kind of messy below when adjusting colors). Here's my finished image without texture:
So, grab your texture image - this can be an interesting piece of paper that you scanned, or just about anything. Mine came courtesy of RobG, but there’s a really nice free texture library here: http://mayang.com/textures/
Here it is 'in action':
Cut out a piece of the texture that matches the size of your picture and paste it into a new layer on top of your image. It should cover it up. Set your texture layer style to 'overlay':
Step 4: Adjust your texture. You should change aspects of the texture layer, like hue, saturation, and opacity. Also, you may want to make some adjustments to your base image now that it's got this overlay - I've upped the contrast and made some color changes. Having the picture as a single layer really makes this process a lot easier.
And it is finished. Hope that was helpful to some people.