I'm not sure if this is true for all images in TH, but the colored eggs for the current promotion have text that tells you the color of the egg so that you would know the rarity.
I made this thread before that promo began, I mean the general 'gem' icons we see that indicate prize rarity. I would also argue that hover text is not enough and the rarities should be visually distinguishable as well (without using color)
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