Reactions in iMessage

If you send the blue iMessages, the heart appears more pink, with a blue background. When you send the green SMS messages, the heart appears slightly darker with a green background.

There is no particular reason for it otherwise.

iMessages Heart:

SMS Messages Heart:

Yes, I can see that, it's because Increase Contrast is activated.

Go to: Settings → Accessibility → Display & Text Size → disable Increase Contrast.

Try going back to the message and see if it doesn't look like the one on my screenshots from my first reply.

From my iPhone, with Increase Contrast enabled:

I can see it. Close the Messages app completely by swiping up to the center of the screen, and swipe the Messages app up and away. Try opening the Messages app again. What do you see?

You're welcome. I noticed on my own iPhone, when I disabled Increase Contrast, the color didn't change back until I restarted the Messages app, so I was prepared for you to ask.