ΔE 17.2, loose: different enough that you should pick on purpose. Light Brown is noticeably darker, much more saturated, warmer than Ratskin Flesh. They are different kinds of paint: Citadel Ratskin Flesh is an opaque base/layer paint and Green Stuff World Light Brown is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Light Brown is noticeably darker, much more saturated, warmer than Ratskin Flesh. They are different kinds of paint.
The same difference applies the other way round: Ratskin Flesh is noticeably lighter, 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 Ratskin Flesh on the colour wheel See Light Brown on the colour wheel
All matches: Ratskin Flesh equivalents · Light Brown equivalents