[Note: Artist Sam Hogg, who illustrated the 2021 A Song of Ice and Fire Calendar, responds to a query about Vhagar's and Arrax's colors as depicted in said calendar.]
I realised while researching that there wasn't any official mention of the colours, so I asked his team :) bronze with greenish blue highlights and bright green eyes.
— Sam Hogg (@Zephyri) January 16, 2021
Arrax was described as pearlescent white with yellow flame, golden eyes and a golden chest :)
— Sam Hogg (@Zephyri) January 16, 2021