Caged Sun is a rare Artifact card from The List (PLST), collector number CM2-178. Current market price: $4.06, TCGplayer market prices via Scryfall.
As this artifact enters, choose a color. Creatures you control of the chosen color get +1/+1. Whenever a land's ability causes you to add one or more mana of the chosen color, add an additional one mana of that color.