Primrose actually reminds us of the old Game of Life program -- the basic rule here is that any color that is completely surrounded by another color will disappear off the board, give you points, and switch the surrounding blocks to itself. In other words, if you place a purple block, and then surround it with yellow blocks, the purple block will disappear, give points, and the four yellow blocks around it will turn purple. Chains can occur if those blocks are then surrounding another color, and so on.
As with many puzzle games, it's a little confusing to understand at first, but eventually you start seeing patterns, and then you're planning out larger and larger clears and scores. Blocks show up on the screen in sets of two, so you only have access to two at most (in fact, it seems as though the game actually chooses to give you colors you don't want just to mix things up), and you can only lay the second block in a pair vertically or horizontally across from the last block you laid. It's a little tough to plan ahead at first, but the game helpfully provides an undo option, so if you lay a block in the wrong place or see a better move, you can undo and try again.
Overall, the game is intuitive to control -- easy to play, but not easy to master. The graphics are simple -- neon colors (with an option to turn on symbols for the colorblind) against a black background, and the sounds are minimal but effective. The game also has networked high score boards, and you can even watch replays of games on the boards, so if someone got a crazy high score, you can hit the Play button and see how they did it.
It is a fun puzzle game with a pretty familiar feel (in addition to the Game of LIfe, it borrows a feel from Go as well as Lights Out). It's not nearly as artistic as Passage, but for most gamers just looking to match blocks and earn points, that's probably a good thing.
Rohrer says the app will be up for $2.99 -- if you're in the mood for a slower-paced puzzle game, it's probably exactly what you're looking for.