Blendoku (Free) is a game you might not immediately recognize as such. I didn’t. Where, I wondered, is the challenge in lining up five colours according to hue? You put the darkest blue on the right side, the lightest blue on the left, and fill the rest with shades in between. A color theory exercise, perhaps, but a game?

That rather exclusionary point of view stuck with me until I took myself out of the lowest difficulty tier and into the next one up. Now I had to do the same thing – arrange the hues between dark and light blue, but in between there were a few shades of brown, a yellow, and a couple colors that looked distinctly purple. In the middle of all that was a small vertical section, and the hues needed to work in that direction too. Now it took thought and experimentation. Now I could see the game of it all.

Many of Blendoku’s levels can be handled intuitively if you have half a grasp on how color works. You’ll get through them in the few seconds it takes to drag the square color samples from the palette above to the play field below. It’s okay that they’re quick, though. There are 475 of them, all for free. 75 are complex enough that they’re only really playable on a tablet, but even so: 400 levels ranging from dead simple to mind-breaking. Pretty good, no?

I suppose that leads us to the point where you’re expecting a second shoe to drop, but I’ll have to disappoint you. There is nothing to stop you from playing the entire game for free. It’s generous to a fault. You can pay to give yourself a small batch of daily hints, but you don’t need ’em. You can play the levels in any order, so if you get stuck you can just move on. On the other hand, any purchase also rids you of the game’s ads. It’s a pretty game. It deserves to be played without ads.

The name “Blendoku" is a play on Sudoku, and once you delve into the game’s higher levels, you’ll see why. Presented with a screen full of empty boxes and just two or three colors already in place, you’ll experience both the discomfort and excitement that can accompany a blank Sudoku grid. Rather than make numbers count both horizontally and vertically, you’re making colors successfully blend both horizontally and vertically. It feels similar because it’s all just logic—the logic of numbers, the logic of colors. Follow the rules and it will all make sense in the end.

But what are the rules of color? That’s the uncomfortable thing about Blendoku—it starts to make you question how you know what you know. I’m not well-versed in color theory. I know about contrasting and complementary colors, and I know how you blend colors together to make others. I know how to look those things up on charts, especially. But how do I instinctively know if one color has slightly more red than it does purple?

I’m not sure, but I know I’m getting better at it with every minute I keep playing this game. I enjoy knowing I can beat the worldwide average speed at laying colors out on a grid in many levels, even if I don’t know why. I’m very much looking forward to the game’s next update, which reportedly notes whether you finish a level perfectly without making any extra moves. It’s easy enough to fiddle around with the swatches until you can see how they blend. Harder to reach the end result without careful comparison.

There’s no barrier to entry here, with one exception: this isn’t a game that’s ever likely to get a color blind mode. Otherwise, though, anyone can play Blendoku, and play it freely. All you need to be able to do is drag a square from one position to another. After that, it’s all fun, games and beautiful color. Why not take a look?