Today I'm gonna show you how to do this!
If the image isn't clear, try clicking on it
The image is pretty self-explanatory, but I'll explain a little more into detail.
1. Once you've gotten your line art make sure it's on a white background. In the image there, you can see my white background is on a separate layer from the line art. (crash course on layers here) This isn't really needed, but I like doing that. If it's scanned artwork, just make sure it's at least mostly white and black. Adding color to this can make it look weird, so I suggest not to. OR if there is color to the image, try making it a black and white image through Photoshop's color settings.
2. Create a new layer. Use Gradient Tool on this new layer. Set the layer's type to "screen" "lighten" and "Hard light" can also work with various degrees of color effects. if you don't like how the gradient's colors look on your drawing, simply redo it on the gradient layer until it looks more like how you'd like it. Done!
Notes: along with this, you can even duplicate your line art layer, and slightly blur it. This will create a sort of "halo" effect in the lineart. If it's scanned work, make sure you set this new blur layer as "overlay" it'll make all the white transparent. If "overlay" doesn't happen to work right, try a different setting.
