While Sudoku boasts a million and one insane variants in the back of the Sunday papers now, you won't have played one like Blendoku before, which ditches the numbers for an art class in colour theory.

The developers at Lonely Few have served up a puzzler with a twist, and hundreds and hundreds of levels that are entirely free to play. It's just a pity that the experience often feels like browsing for colour swatches.

I can see a rainbow

The concept is simple: you have to fill the grid - initially a line or small square - with the supplied coloured tiles. Each is a blend of its adjacent colours, so it's up to you to plonk them in the right point on the gradient. You're on the clock when you're doing so, competing against all other players across the globe for the best time and the cleanest attempt.

Over the course of hundreds of levels, things escalate from simple grids to arcane labyrinths of forking hues, and grabbing those Perfect badges becomes much harder when there are upwards of 20 shades for you to place with the most paltry of clues.

You might be tempted to take a peek at a clue - one a day or pay for extra - but you might also just give up.

Like watching paint dry

Despite Blendoku's name, it has no real relation to Sudoku.

Sudoku's appeal lies in mastering its rigid logic, which can be explained and reverse engineered. Blendoku doesn't constitute a logical test so much as an ocular one - you don't calculate the answer so much as discern it.

There are some basic rules to learn in stages with grids on two axes, but they're hardly challenging. For the most part, Blendoku feels less like a puzzle than it does a virtual trip to B&Q.

That's not to take away from its casual appeal entirely. This is an innovative, accessible, and fundamentally beautiful game - it's just not a very deep one.