ΔE 19.0, loose: different enough that you should pick on purpose. Red Glaze is much more saturated, cooler than Guilliman Flesh. They are different kinds of paint: Citadel Guilliman Flesh is a one-coat contrast-style paint and Two Thin Coats Red Glaze 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 19). Red Glaze is much more saturated, cooler than Guilliman Flesh. They are different kinds of paint.
The same difference applies the other way round: Guilliman Flesh is much more muted, warmer.
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 Guilliman Flesh on the colour wheel See Red Glaze on the colour wheel
All matches: Guilliman Flesh equivalents · Red Glaze equivalents