I'll show you how to use the Lock Alpha tool in Java, how to make rainbow lineart using Lock Alpha and my favourite part of Lock Alpha, staying inside the lines. :)
So, first I drew a smiley face. :) You can draw whatever you want. You can even draw a line. It will still work with this tutorial.
You should check your Lock Alpha. I'm sure alot of people know where it it but if you don't, there's a red arrow and it's highlighted.
So, now that your Lock Alpha is checked off, get your Airbrush tool, and use the first colour of the rainbow. :]
NOTE: It can be any colour you want, but I perfer red. ;]
Now, Airbrush a little section or your [insert object here]. Mine is red on the type like shown in the image. Now use all the colours of the rainbow until you get a rainbow effect. :D
Now your smiley face is rainbow! Hmmm. . . It looks kind of plain. Let's give it a background. Any Dark Colours will make the rainbow pop out. :P
So, I added a background. :) I used brown, black, gray and dark purple. :D Now onto colouring.
Let's us the smiley face from step one again. :D So now, it's just a black smiley face. So go the layers and if you had a layer underneath, good on you. If you don't add a layer above the [insert object here] and drag layer 2 under layer 1. Then go the modes [Normal, Multiply, Dodge, Burn ect] and press Multiply. Then you can colour under layer one. :)
So we colour under our smiley face with yellow. :D Typical smiley face colour. Now press Lock Alpha and use a different shade of yellow. Darker is best. :P
Now we use that darker colour and shade. I don't use Airbrush for this. I use the pencil, then I use the blend tool. :) Now to use lighting. :D
Use the lighter yellow in places that are "in front" Of the shading. And the classic shine. ;) Well. That's my tutorial on how to use the Lock Alpha. You can use it for many things. :P