vierbit is right. 8 palettes, 4 colours each for tiles. 8 palettes, 4 colours each for sprites, but here 1 colour of each palette is transparency, so you only get 8 3-colour palettes for sprites.
Also keep in mind that there are no layers on the GBC, a tile is either in front of the a specific sprite or behind it. (I think one colour of a tile can be set as transparency as well so that you could have 3 or so pixels high grass in a platformer which goes infront of the sprite.)
Also ALL Nintendo handhelds with colour from the GBC up to the DS have 15 bit colour, 5 bit per channel. In Cosmigo Promotion you can set the colourdepth per channel. In effect here it means you can only have hex values of
00, 08, 10, 18, 20, 29, 31, 39, 41, 4A, 52, 5A, 62, 6A, 73, 7B, 83, 8B, 94, 9C, A4, AC, B4, BD, C5, CD, D5, DE, E6, EE, F6, FF
for each channel.
The mockup you have is quite nice, vivid style, I agree with the crits you got on the top bit of the house and the player colours as well.
Also you have quite a lot of errors, in terms of colours per tile and total amount of palettes. Good work so far tho, just needs more polish if you want it to be true to the GBC standards.