If a red and yellow mixture turns browner than expected what is the most logical correction?

Correct answer: Make a fresh mixture using clean orange-biased red and yellow

The governing concept is subtractive pigment mixing and colour bias. An unexpected brown or muddy orange commonly results from blue contamination, dirty tools, or pigments whose biases oppose the desired orange. Starting again with clean tools and a red and yellow that lean toward orange reduces the unwanted third-primary influence. Adding blue, all primaries, black, or grey would make the mixture duller.