I bought a small pack of page flags recently, and I've already used them more than some of the sticker sheets sitting in my drawer. I stick one on a page whenever there's something I need to come back to, then move or remove it when I'm done.
I still buy decorative stickers, but I'm trying to be more selective about those. Page flags are something I know I'll actually reach for.
I'd probably use two colours at most: one for pages I need to find again and one for unfinished things. That's different from using the flags without a colour system, but I'd lose track if I made it any more complicated.
I use placement for the distinction: outer edge means find this page again, top means there's an action to handle. Then the colors can just be colors, one less tiny system to maintain.