Hot take: I used to prefer Luigi's color scheme in Super Mario Bros. Deluxe as it's more in-line with his modern appearance, but now I like the original NES design more as it makes the character feel more unique and less like another Mario.
Also Yoshi is the GOATshi

@Kc64 SMB2 uses an overlay sprite for the eye color.

A lot of people are saying that Yoshi can't have four colors as he does here.

The GBC, like the NES, supports three colors per sprite palette. Larger sprites, or metasprites, are made up of several 8x8 (or in this case, 8x16) sprites, and each of the individual sprites in one metasprite can use different palettes.
For example, consider the Koopa Troopa in Super Mario Bros 3. It uses one palette for the skin and another for the shell, which is 8 pixels wide. The fortress coloration makes it easier to see which palettes are used where. Yoshi is arranged in a similar manner here, which is how he can have four colors.

I'm making a fangame in GB Studio, and using this coloration method lets me make Peach's hair yellow without making her look jaundiced.

@Luigifan1115 , yes.

@Diyamund do you mean mood or mod and in THAT case why is that here?

Wait smb2 has 4 colours so it is ok

Yoshi has 4 colours nintendo you can't do that on ne

Do we really need all the possible palettes set up as if they were extra Mario brothers?

so that's where the Mari0 Yoshi came from...

@Ayumi Background isn't black though, and the illusion there kinda required they be surrounded a bit more than that.

I would argue Yoshi uses 3. The dark green can just be replaced with black like Bowser's cuffs in the original.

yoshi uses four colors agggghhhhhhhhh nintendo why

There's an error on Fire Mario's firing animation.

peach's second sprite is a mood

The extra sprites for Peach and Toad look fan-made, honestly. They're not that bad though.

I would prefer these sprites for Luigi than the ones on NES