The Interior
The interior for the 1959 Eldorado Biarritz featured top-quality cardiff and florentine grain leather for seats and door panels. It was available in 7 different colors:

Bronze Metallic bucket seats interior to match persian sand exterior

Red bucket seats interior with seminole red exterior

White interior

Blue Metallic

Green metallic interior with hampton green exterior

black interior
Special Order Interiors
Unlike with the Series 62 Convertible, no standard two-tone interiors were offered. If you see a 59 Eldorado with a two-tone interior, its either a non-correct restoration (like many are...) or a very rare special order interior like the two following cars.
The next three photos are courtesy of Yann Saunders, aka"Mr. Cadillac". He writes:
"These photos depict two "Special Order" interiors. I took them in Indianapolis in the late seventies. The owner of the white car with the blue/white interior said he knew of another Argyle Blue Biarritz with an "SO" blue/white interior, but with blue bolsters and white seat and seat back inserts. Unfortunately I never got to see that car, nor photograph it"

Special order interior Photo, courtesy Yann Saunders, aka"Mr. Cadillac

Special order interior Photo, courtesy Yann Saunders, aka"Mr. Cadillac

Special order interior Photo, courtesy Yann Saunders, aka"Mr. Cadillac