Light Red vs Amaranth Red

How close are they?

ΔE 15.9, loose: different enough that you should pick on purpose. Amaranth Red is noticeably darker, warmer than Light Red.

Can I use Amaranth Red instead of Light Red?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Amaranth Red is noticeably darker, warmer than Light Red.

Can I use Light Red instead of Amaranth Red?

The same difference applies the other way round: Light Red is noticeably lighter, 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.