[rodw]

More colors seems like the obvious way to increase the difficulty as the user levels-up. With only four colors, random play is quite likely to get a match--the odds of a randomly selected tile matching a given tile is going to be 1 in 4.

From a cursory look (my math may be a little off) it seems like the odds of a tile matching at least one of its 4 cardinal-direction neighbors is ~57% (20 of 35 order-independent variations), and the odds of a tile matching 2 or more of its randomly selected neighbors is ~28%. In other words, with just random play you're going to get a match out of every `+` shaped collection of 5 tiles nearly 1/3rd of the time. Factoring the three-in-a row possibility the odds of a match will go up from there. (And since a match will eliminate several tiles of given color, the odds of getting a match on the "chain reaction" must also be quite high.)