Builderboy once gave me an awesome little tutoring session on how it works. Let's see if I can dig it up...

Code snippets in spoiler tags are messed up in Chrome. Yay for one line source viewing!

I don't know anything about Axe... Does it allow recursive functions ? If yes, it's really easy...
Anyway, Wikipédia is really interesting about that.
http://en.wikipedia.org/wiki/Flood_fill

Aw, too bad.
So I guess you'll have to use use the fixed memory way (right-hand fill method) to do this.
But it is quite slow with complex shapes.
So, good luck

I've done flood fills, though, it wasn't for painting or anything, but here's how I did it.

1. Make a buffer the same size as the number of pixels in your drawing area and fill it with 00's.
2. Take all the spaces eligible for filling and mark them with 01 on the new buffer.
3. Mark the space that was picked by the cursor with a 02.

This part is a loop:
4. Search the buffer for a 02
5. When you find it, check the 4/8 (depending on implementation) squares around the 2, if they are 1, mark them with 2, if they are 0, do nothing.
6. Mark the current square with a 03
7. Repeat this part

Flood fill is over when there are no more 02's left.
8. Find all of the 03's and fill them in back on the original picture


This technique should work as long as you don't have a massive buffer. But even if you did have a buffer the size of the screen, a full buffer for this would only require 6,144 bytes. Then, since this method only has values between 0 and 3, you could potentially store 4 pixels to a byte and get your size down to 1,536.

If space is not a problem, I also have an optimization to make this technique run faster. Checking the edges can slow the process down because you always have to figure out whether you need to check all 4 sides. This can be avoided by making your buffer 1 extra row wide and 2 extra rows tall. Let's say you have a 16x16 picture. If you make your buffer 17 pixels wide, you'll have 1 pixel boundary along both edges, this would mean you never have to check left/right boundaries. Then, if you give the buffer an extra row on the top and bottom, you can check the whole thing without fear of going over an edge. Just remember, you obviously don't want to perform your buffer operations on that 17th row, just skip it when checking for 02's.


Edit:
    After I wrote this, I wrote a tutorial on it. I've decided that instead of making posts like this, from now on I'll just write tutorials and link them