Light Magenta vs Old Rose

How close are they?

ΔE 25.6, loose: different enough that you should pick on purpose. Old Rose is noticeably darker, more muted, warmer than Light Magenta.

Can I use Old Rose instead of Light Magenta?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Old Rose is noticeably darker, more muted, warmer than Light Magenta.

Can I use Light Magenta instead of Old Rose?

The same difference applies the other way round: Light Magenta is noticeably lighter, more saturated, 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.