Walker introduced us to the monochromatic delights of indie puzzle-platformer (increasingly the de facto indie flavour, in the same way the de facto indie film flavour is interlocking vignettes about miserable peoples' miserable lives) Shift a while ago. At least I think that's what he was posting about, but of course I didn't read it because it was by Walker.
The third one's out now, and it's expertly pushing the lovely concept (obstacle navigation via colour-inversion, sort of like Lost Vikings via Ikaruga) into something new.
No longer a series of standalone rooms, it's instead one long adventure. A switch flipped in one room might open a door several rooms back, or picking one exit door over another might lead you down an entirely different path. This involves a fair amount of Metroid-style backtracking, but rewardingly so - you're genuinely plotting out an elaborate puzzle-path.
The cheerily shallow plot borrows heavily from Portal, but hey - there's nothing wrong with thrilling escapes from mad science experiments, right? It's all exceptionally polished and a right old treat, the two-tone graphical approach proving as delightful as a monkey eating an ice cream, even third time around.
Honestly don't worry if you've not played the first two instalments, by the way - there's a quick'n'dirty tutorial up front that tells you all you need to know whilst still being cannily streamlined into the main adventure. Go play.