Why may a foliage group turn brown when colours mix excessively in wet-on-wet watercolour?

Correct answer: Too many opposing colours mixed uncontrollably on the wet surface

When several opposing or near-complementary pigments mix on a wet surface, the mixture can lose much of its colourfulness and look brown or muddy. This is why excessive, uncontrolled mixing can dull a foliage group. Paper thickness, brush size, or a light green alone does not explain this effect.