Titan Green Pale vs Stonewall Grey

How close are they?

ΔE 19.6, loose: different enough that you should pick on purpose. Stonewall Grey is noticeably darker, more muted than Titan Green Pale.

Can I use Stonewall Grey instead of Titan Green Pale?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Stonewall Grey is noticeably darker, more muted than Titan Green Pale.

Can I use Titan Green Pale instead of Stonewall Grey?

The same difference applies the other way round: Titan Green Pale is noticeably lighter, more saturated.

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.