Terrestrial Titan vs Dusky Grape

How close are they?

ΔE 15.7, loose: different enough that you should pick on purpose. Dusky Grape is much more saturated, warmer than Terrestrial Titan.

Can I use Dusky Grape instead of Terrestrial Titan?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Dusky Grape is much more saturated, warmer than Terrestrial Titan.

Can I use Terrestrial Titan instead of Dusky Grape?

The same difference applies the other way round: Terrestrial Titan is much more muted, 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.