Solar Flare vs Medium Yellow

How close are they?

ΔE 22.3, loose: different enough that you should pick on purpose. Medium Yellow is a little lighter, more saturated, cooler than Solar Flare.

Can I use Medium Yellow instead of Solar Flare?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Medium Yellow is a little lighter, more saturated, cooler than Solar Flare.

Can I use Solar Flare instead of Medium Yellow?

The same difference applies the other way round: Solar Flare is a little darker, 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.