Although the word "retablo" has religious connotations, in your context it may not matter so much if you don't translate it literally as "altarpiece". However, I would personally use "polychrome" instead of "multi-coloured", as it looks like we are referring to a work of art, and this would not be so obvious if you used "multi-coloured".

See DRAE definitions below (I don't think entry number 3 would be relevant in this context):

http://lema.rae.es/drae/?val=retablo:
1. m. Conjunto o colección de figuras pintadas o de talla, que representan en serie una historia o suceso.
2. m. Obra de arquitectura, hecha de piedra, madera u otra materia, que compone la decoración de un altar.
3. m. Pequeño escenario en que se representaba una acción valiéndose de figurillas o títeres.