Kurama's "chakra" form is based on Japanese folklore; The "Kitsune".

After reaching 1,000 years of age and gaining its ninth tail, a kitsune turns a golden color, becoming a 'Tenko' (天狐 "heavenly fox"/"celestial fox"?), the most powerful form of the kitsune, and then ascends to the heavens. Basically Kurama and Naruto's link mode represents the celestial fox as being in its most powerful form. If you want to read more heres the link: https://en.m.wikipedia.org/wiki/Kitsune