Light back causes a general reduction of the body colour of the bird, while keeping the black markings intact.
This is a sex-linked recessive mutation.
The light back mutation is situated on the same gene as the chestnut flanked white (multiple alleles). A male zebra finch split for both light back and chestnut flanked white will visibly be a light back.
The colour separation of head/neck and back/wings is required on the European continent.