Thunderous Blue vs Pastel Blue

How close are they?

ΔE 15.5, loose: different enough that you should pick on purpose. Pastel Blue is noticeably lighter, more muted, cooler than Thunderous Blue.

Can I use Pastel Blue instead of Thunderous Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Pastel Blue is noticeably lighter, more muted, cooler than Thunderous Blue.

Can I use Thunderous Blue instead of Pastel Blue?

The same difference applies the other way round: Thunderous Blue is noticeably darker, more saturated, warmer.

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.