I loved it! This was a pleasant way to start my morning. It was not very hard, and that was fine with me because it was fun and cute.
There were some chances you missed to create visual variety. The aesthetics of the desert could change with the music (e.g. at night, in the rain, etc.). The cactus could frown when it is stuck in a block, or melt when it is drowned in a waterfall.
The music was adorable. Did you write it? My only suggestion is I think there should be an additional separate track for the title screen, different from the first levels. Initially, I thought that short loop was going to run through all twenty levels, and inwardly decided to shut off the sound around level 5. I was grateful when the music changed, though I like that track a lot.
The gameplay could be easily expanded out of one screen, which I think is what accounts for the relatively easy difficulty level. You could add more colors and have the cactus track larger levels, forcing the player to genuinely remember the structure of the colors just above or below the visible screen area. You could also exploit the cactus's falling time, making the player change blocks from on/off in a tricky sequence as it fell through the air.
Thank you for creating and sharing this!