Grotesque Green vs Titan Buff

How close are they?

ΔE 16.7, loose: different enough that you should pick on purpose. Titan Buff is noticeably lighter, more muted, warmer than Grotesque Green.

Can I use Titan Buff instead of Grotesque Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Titan Buff is noticeably lighter, more muted, warmer than Grotesque Green.

Can I use Grotesque Green instead of Titan Buff?

The same difference applies the other way round: Grotesque Green is noticeably darker, more saturated, cooler.

ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.