Hykey Yellow vs Ivory RLM05

How close are they?

ΔE 17.3, loose: different enough that you should pick on purpose. Ivory RLM05 is noticeably darker, more saturated, cooler than Hykey Yellow.

Can I use Ivory RLM05 instead of Hykey Yellow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Ivory RLM05 is noticeably darker, more saturated, cooler than Hykey Yellow.

Can I use Hykey Yellow instead of Ivory RLM05?

The same difference applies the other way round: Hykey Yellow is noticeably lighter, 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.