There are several factors why some fat in beef animals is yellow; however, no body could nail down the exact reason or reasons;

a) breed

b) forage

c) seasons of the year

d) age

e) technical answer; Animals which tend to be more omnivorous, including humans, are less specialized than herbivores in breaking down dietary carotine, the photosynthetic pigment which makes carrots orange and butter yellow.

The carotene is stored in the fat cells, giving them a yellow color, where they can be converted into biologically useful (and colorless) retinoids (vitamin A) as needed.

Cattle and other major herbivores, however, convert the carotene into retinoids directly before storing them in fat, making the fat appear white instead of yellow.

Probably the most commonly consumed animal with visibly yellow fat is chicken.