The concept made sense enough to me. The easy levels seemed to usually take me between 2 and 3 minutes; I did one medium level that took a little more than 15.
I like the simplicity and cleanliness of the interface. A couple times I thought I wanted a sort of "negative" marker, to mark that I knew a block couldn't be a particular color, but that would probably over-complicate the interface too much.
It was easy enough to see in the normal browser area:
When I tried "fullscreen" mode anyway, it didn't expand to fit the screen (in Firefox 140.13.0esr, 1600x900 display).
When leaving fullscreen mode, it wound up clipped by the available space in the browser.
