Don't know if it should be read as まいろ, まあいろ, あさいろ or in some other way. Couldn't find it in any dictionary.
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It's read like a regular compound word, あさいろ
Here's a passage from Aozora Bunko with furigana:
I'm not 100% sure, but the trend seems to be attaching いろ directly to the standalone word. Going from this, I would assume that 麻色 is read as あさいろ.
Apologies for not being able to provide a confident answer.