Nice game, I like the idea.
However, it's quite tedious to cycle through all colors. I think, it wouldn't be bad if you could use the mouse wheel (forwards and backwards) or press a corresponding key. And if the currently selected color is displayed anywhere (on the paddle, for example).
Also, the gameplay might become more interesting, if the distance between the ball shooter and the player paddle is bigger (i.e. smaller, slower balls), while there may be multiple balls at a time. If they appear in irregular intervals, it can become quite challenging. Another possibility is to add power-ups or items, like wall fortification/repair, shoot multiple "anti-balls", freeze the enemy movement for a second, ...
Just some thoughts, I'm sure there are a lot of possibilities to expand Ball Defender (given you want to continue developping it).
