In this random photo, Reddit user Goobz24 visually depicts the color distribution from a single party size bag of Skittles. While a single bag is clearly not representative, a study conducted in 2002 analyzed the color distribution of 504,032 Skittles and found the distribution to be almost dead even [source].

In the bag above, the breakout was as follows:

Red: 80 (20.8%)
Orange: 74 (19.3%)
Yellow: 74 (19.3%)
Green: 91 (23.7%)
Purple: 65 (16.9%)
Total number: 384