[DanJ]

There doesn't seem to be a color restoration hack for this game anywhere. How tedious would it be do that myself? I want to know if it'll be worth the effort.

[ShadowOne333]

I tried that myself, but the palette data seems to be ordered differently than the SNES ROM.
You could give it a try of course, it is absolutely possible, just really time consuming.

I say give it a try and if you can at least restore the title screen colours, you are going the good way.

[Googie]

I just wanted to let you know DanJ, that'll be pretty cool to see.

[Chronosplit]

I just wanted to let you know DanJ, it's been done. you can get it here.

That's for the first one which is Super Mario Bros. 2.  He's asking for Super Mario Advance 4, which has Super Mario Bros. 3.

The whole thing was basically a series of GBA ports with Mario Bros. included as an extra.

[Googie]

Yeah I realized the mistake I did and fixed the post, my bad. 

EDIT: Btw, does SMA4 has annoying voices? I'm just taking a shot in the dark. ROM hunting for me, right?    

[Axiphel]

The difference between the GBA version and the SNES version is just a slight difference in brightness. (aside from the obvious resolution differences...) It's not really worth the effort which is why it probably hasn't been done but that's up to you. You could achieve SNES color with just a slight reduction in brightness in the emulator.

GBA

SNES

[Jorpho]

There were actually a surprising number of thoughtful changes made to the level layouts to accommodate the new resolution.
http://www.themushroomkingdom.net/smb3_snes2sma.shtml

By the way, is there a patch yet equivalent to the Wii U Virtual Console version, i.e. that neatly adds in all the e-Reader content?

[Axiphel]

By the way, is there a patch yet equivalent to the Wii U Virtual Console version, i.e. that neatly adds in all the e-Reader content?

Yes, there is. You could find a patch on gbatemp. Can't be hosted here because it's Nintendo's content I guess.

[Animation Guru]

The difference between the GBA version and the SNES version is just a slight difference in brightness. (aside from the obvious resolution differences...) It's not really worth the effort which is why it probably hasn't been done but that's up to you.

It seems mostly identical to me.  Is it possible that the game adjusts the palette when it detects that it's being played on Gameboy Player?