The Rainbow Costume is a retired costume with red, orange, yellow, green, blue, indigo, and violet. It was first released in July 2005 in Account Upgrades and was re-released in March 2017. Putting this costume on certain marapets will make it look like a rainbow with black, white, and aqua being the secondary colors!
Rainbow Pets
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Attach a Rainbow Zax to a Rainbow pet then view the minipet.