Highlord Blue vs Indigo Blue

How close are they?

ΔE 21.9, loose: different enough that you should pick on purpose. Indigo Blue is noticeably darker, much more muted than Highlord Blue.

Can I use Indigo Blue instead of Highlord Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Indigo Blue is noticeably darker, much more muted than Highlord Blue.

Can I use Highlord Blue instead of Indigo Blue?

The same difference applies the other way round: Highlord Blue is noticeably lighter, much 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.