Underbelly Blue vs AII SV. Gol Lt Blue

How close are they?

ΔE 20.5, loose: different enough that you should pick on purpose. AII SV. Gol Lt Blue is a little darker, much more saturated than Underbelly Blue.

Can I use AII SV. Gol Lt Blue instead of Underbelly Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). AII SV. Gol Lt Blue is a little darker, much more saturated than Underbelly Blue.

Can I use Underbelly Blue instead of AII SV. Gol Lt Blue?

The same difference applies the other way round: Underbelly Blue is a little lighter, much more muted.

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.