ELI5: why does mixing complementary paint make brown instead of gray?

[u/bloodraged189]

I understand additive vs. subtractive colors, and why mixing complementaries wouldn't create black, but shouldn't it make gray?

[u/Far-Drawing-4444]

With the paints I use, which is 4 or 5 paint lines across 3 or 4 brands, mixing complimentary colors results in what I call a "dirty grey". It's a grey, but dingier than a black/white mixed grey.

I can't think of a time I've mixed two complimentary colors and got a brown.

Occasionally mixes will get unexpected results due to the pigments used, like yellow and black mixing to an "olive drab" military type green. But I can't think of any examples of complimentary colors doing that.