To straighten things out:
Everything is fine as far as color limits. Sprites can have 4 colors (including transparency). If anything, there may be a rule I'm unaware of like "only x amount of colors on-screen at once" or something, but even then I think it's fine.
The only thing out of style are the sizes of the sprites; the dimensions should tecnically be multiples of 8, but I wasn't too worried about it.
Look more closely; everything only has three colors.
Luigi (including poltergust 3000): Blue, green, tan
Ghost: Light blue, orange, red
Curtains (inlcuding window and stand): Red, dark red, dark blue
Wood section of wall: Brown, yellow-orange, black
Floor (including carpet): Dark red, orange, yellow
Paintings: Yellow-orange, brown, black
Chandlere (including suspending chains): White, yellow-orange, brown
Vaccuum sucking tunnel: Dark grey, grey, white
Wall paper: yellow-orange and orange
Heart: Red, dark red, white
Coins: Brown, yellow-orange, white
Pearls: Dark grey, grey, white
Status texts: Each use two colors
Speaking text: white
Gameboy (inlucding E. Gadd image): Dark grey, grey, white
Also, every color is taken from official SMB sprites. If you
think there are more colors, then I did it right. :P
If anything, it looks too advanced because most NES games don't have a complex background.
Thanks for comments all.