What is this most likely weed, though with interesting foliage color?
The only specimen in my yard, growing under a mespilus tree.
The flower is NOT of this plant, but the photo is for stem details:
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It is Euphorbia helioscopia. One of the first flower in spring. And one of the few green flowers (maybe it is yellow). Euphorbia have strange inflorescence and flowers (and I'm not yet sure I understand it). The spatulate leave and the large inflorescence (and the period) tell me the species. There are many wild Euphorbia in Europe.
And I'm starting thinking you are my neighbor. The next flower you will ask. it will be Ornithogalum or Cruciata/Gallium.
It's one of the Euphorbias, common name spurge - but I'm not sure which. Usually, with the ornamental forms, there's a basal clump of leaves, but I can't really see any at the base of yours. Image here of a similar one, with a basal clump of leaves https://www.ballyrobertgardens.com/products/euphorbia-redwing-charam. Euphorbia is a huge group of plants with some flowering early during spring, others early summer and other varieties late summer/autumn. There are also a few 'weed' ones that pop up on their own - Euphorbia martinii does, as well as the ubiquitous euphorbia lathyris, though the latter is not similar to your plant at all.
The white flower is unrelated to this plant - if/when you cut the Euphorbia, be careful of the sap, it can irritate the skin.