How to keep terracotta pots from discoloring?

I have a small lily planted in a 6" terra cotta planter. The planter has strong colors on it. Is this mold/mildew from moisture? How can I keep the planter from doing this? It is indoors right now because it is snowing outside. The bottom of the planter dark is soil. I am mainly talking about near the top of the planter- it is beige in color and looks like it has texture to it. Any suggestions are greatly appreciated.

q how to keep terra cotta pots from dis coloring
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