That sounds like the "Rainbow BG" color in the Lab tileset - you can only select it if you set the room to manual or multi mode.

The tower tileset is not normally selectable in custom levels, but you can customize the graphics assets for your level to make other tilesets look like the tower.

Which level did you see the rainbow background in?

Sorry for the late reply.

So you're talking about the room PARTY TIME, right? (2:06 in the video.) That is actually the Lab rainbow background, not the Tower one.

You can also look at the level file here: https://distractionware.com/forum/index.php?topic=1626.0

I did read your note; the rainbow room I see in Cloning Technology is really the Lab background.

To enable it, in a room with the Lab tileset, set the mode to either manual or multi (not auto), then cycle through the colors until it says Rainbow BG.

You can change the mode with the button that says something like "Mode: auto" or by pressing F10 once or twice.

Making the tileset change color is much trickier, but technically possible with advanced scripting techniques - if you make multiple versions of the same room with different colors, there's the internal scripting command `gotoroom(x,y)` which you could run on some kind of timer. But it requires quite some explanation and layers of background knowledge; how much do you know about (internal) scripting?