The orange parts of the pickaxe are receiving a faint dark blue light from somewhere while outdoors. It is only visible when the pickaxe is not in direct sunlight, eg. turning the player character in a direction where she casts a shadow on the pickaxe, and most noticeable along the edges, kind of like faint rim lighting or edge highlight. At night, it is barely visible but still there.
There is a similar highlight indoors, but there its white, which matches the white indoor lighting, so it looks correct there. So I'm guessing this is intentional, only the light source used outdoors for it is set incorrectly.
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