It will spawn a reb_block at each red_spawn, if you change your event like so:
On key Z pressed
-sub-event- red_spawn: For each object
and place the create object action in that sub-event.
I think what you could do instead of spawning/unspawning objects is to use "Change collision mode" on your red, green, and blue objects. It's in the actions for your objects.
Set it to "none" to make them non-solid. Make them per-pixel again to be solid.
However, I do not think there is a way to remove the actual solid attribute. Not yet, at least. (I saw mention in )
Collision mode none will, as far as I know, stop "overlapping at offset" and such things from working. Still, for what you have so far, changing the collision mode should work okay.
It might get tricky if you're going to need to detect when the player enters another color, for that dropping-down behavior.. Maybe a combination of spawning solid blocks on top of a non-solid background, but the background would have per-pixel set, so you can detect when the player overlaps them. Sorry if I misunderstood anything. I'm sleepy.