Blue Base vs Orchid Light

How close are they?

ΔE 26.6, loose: different enough that you should pick on purpose. Orchid Light is noticeably lighter, warmer than Blue Base.

Can I use Orchid Light instead of Blue Base?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Orchid Light is noticeably lighter, warmer than Blue Base.

Can I use Blue Base instead of Orchid Light?

The same difference applies the other way round: Blue Base is noticeably darker, 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.