You only see the colors on messages you send. OP is implying that you wouldn’t know what “color” the conversation is unless you’re actively replying to the spammer.
You can still tell whether it is an iMessage or text message without replying and observing the color. Long-press on the incoming message. If the menu shows: Reply, Copy, Translate, More… then it is an iMessage. If the menu shows: Copy, Translate, More… then it is a text.
That's cool, didn't know that Apple didn't try to somehow shoehorn threads into SMS (like they currently do extremely poorly with tapbacks and stickers)