Pixels $=512\times512=262{,}144$.
Each pixel has 3 bands $\times$ 4 bits/band $=12$ bits.
Total bits $=262{,}144\times12=3{,}145{,}728$ bits $=$ \(\boxed{31{,}45{,}728}\) (as written with Indian commas).
(= \(3{,}145{,}728/8=393{,}216\) bytes $\approx 384$ KB.)