Cute tool! :D I enjoy its simplicity :)

I noticed when you change the slider values, it'll actually draw the pixel behind it :0 that is probably not intended, but if, while dragging, you then move your mouse up, you can create a really cool gradient :D  Really got my creative juices flowing :D

That you made the right mouse button erase and middle mouse button a color picker, while great, took me some time to realize, maybe you could have added a small symbol next to all the mice in the top right :0  While you're there, a save button could be a good addition! 
Also the  scroll to zoom seems to be inverted :\ Btw why do I have to use the Arrow keys to move :( it's so uncomfortable, I just want to use WASD D:

Some cool features you could add could include hot keys for different brightness/saturation levels (e.g. pressing 1 gives makes it dark and pressing 9 make it bright). A preview of the current color would also be cool (maybe even make it so it shows the hex value and that way you can input a different one) :D

But all in all, Great job! I love that "x" button  XD