Yes, colors are random because that's the whole point, you need to get 3 matching colors in a group in order to advance to next floor.
If I ever do a full version of this game, I might add full sandbox mode, where you could choose different colors you want to spawn, but for this game jam, that kind of gameplay felt too boring for me. Until then, you can mod the save game and change colors, they are represented in RGBA.
I just added deterministic random (Y key) to the game, which makes save-scumming a bit easier. Enable it, then press J key to save and whenever next spawn is wrong color, press L key to load previous state, waste the spawn to the side and click the right spot again.