Reminds me of red orach.

Yes, but turns completely green.

Some of mine did that.
Let it grow a bit and post another picture.
Or, if you don't want a plant in that spot, dispatch it now.

Orach is an annual. Judging by the thickness of the stems these sprouts look like a perennial emerging in the spring. Is there a big fat root under there? Maybe Mirabilis jalapa aka Four O'Clocks. They also seed around which would account for the presence of seedlings alongside.

Thank you!!