ΔE 16.9, loose: different enough that you should pick on purpose. Summersea Blue is a little lighter, much more saturated, warmer than Thousand Sons Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Summersea Blue is a little lighter, much more saturated, warmer than Thousand Sons Blue.
The same difference applies the other way round: Thousand Sons Blue is a little darker, much more muted, 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.
See Thousand Sons Blue on the colour wheel See Summersea Blue on the colour wheel
All matches: Thousand Sons Blue equivalents · Summersea Blue equivalents