Rather than fix the color of the first slot, just roll the second slot so it's the same color as the first. Let all attributes be possible on the first slot. The end result will be the same - no loot with all three colors. However, this will still allow for hybrid pieces - e.g. Golan with blue core + 2 yellows. If someone wants a three-color piece, they can then re-roll one of the non-core attributes.
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