Royal Blue vs Indigo Sky

How close are they?

ΔE 16.3, loose: different enough that you should pick on purpose. Indigo Sky is much more muted, warmer than Royal Blue.

Can I use Indigo Sky instead of Royal Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Indigo Sky is much more muted, warmer than Royal Blue.

Can I use Royal Blue instead of Indigo Sky?

The same difference applies the other way round: Royal Blue is much 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.