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.

The text input box will also say whether it is a text message or iMessage before you start typing anything.

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)

Messages you receive are always black text on a grey bubble. What part of this was confusing?

As other have mentioned, you can tell if it's iMessage or not in a few different ways, the send button color, the "Text Message" or "iMessage" label interspersed in the conversation, etc, but if you want to talk about the color of the bubbles, that only applies to outgoing messages.