A target colour from two base paints is a ratio problem, not a guess — the same two paints in a different ratio produce a completely different, unrepeatable shade.
Without measuring by volume, matching a mixed colour a second time (for touch-ups or a second room) becomes nearly impossible — tiny ratio differences shift a colour more than most people expect.
Volume ratios assume the pigments have similar tinting strength, and they emphatically do not — a strong phthalo blue will dominate a titanium white at ratios where the arithmetic says otherwise. Paint also dries a different shade from wet, so the ratio has to be judged on a dry sample.
Estimate with the rule, then check it against the calculator that models it properly.
Open Paint Colour Mixing →A target colour from two base paints is a ratio problem, not a guess — the same two paints in a different ratio produce a completely different, unrepeatable shade. Without measuring by volume, matching a mixed colour a second time (for touch-ups or a second room) becomes nearly impossible — tiny ratio differences shift a colour more than most people expect.