Nice and simple game. Good concept, and a good take on the theme.

Some suggestions:

• Maybe have varying left-right movement of the balloons (just like your vertical speed variations). Right now, all balloons on screen seem to move left or right at the same time.
• There seems to be no penalty for shooting a non-red balloon. So while the score does not increase, maybe have an early game-end for shooting 1, or some number of, non-red balloons?
• There are too few balloons on screen at the same time. Maybe have a few, or a lot, more balloons on screen? Make it a real "Where's Waldo" of finding the red balloons? Or, as Ross suggested below, implement some mechanics, e.g., red balloons have a particular left-right swaying pattern, or a particular vertical speed, so you need to watch out for balloons of other colors matching those parameters, etc.

The constant left and right of the balloons made me a bit sea sick. I do love the theme interpretation. The clicking of the balloons was very intuitive. 

Fun point and click mechanic, it might be good if different colours had different speeds and floating patterns. Great work!

This is a sweet and simple take on the theme. I think a popping sound for the balloons would be a nice addition.