Python 2, 132 131 bytes

def f(n):
 t=n+n/8;s=('vibgyor '*n)[:t];r=[s[~i:]+t*' 'for i in range(t)]
 for l in zip(*r+3*[' '+s]+r[::-1])[::-1]:print''.join(l)

Try it online!

Saved:

• -1 byte, thanks to Jonathan Frech

Python 2, 132 131 bytes

def f(n):
 t=n+n/7;s=('vibgyor '*n)[:t];r=[s[~i:]+t*' 'for i in range(t)]
 for l in zip(*r+3*[' '+s]+r[::-1])[::-1]:print''.join(l)

Try it online!

Saved:

• -1 byte, thanks to Jonathan Frech

Python 2, 132 bytes

def f(n):
 t=n+n//8;s=('vibgyor '*n)[:t];r=[s[~i:]+t*' 'for i in range(t)]
 for l in zip(*r+3*[' '+s]+r[::-1])[::-1]:print''.join(l)

Try it online!

Python 2, 132 131 bytes

def f(n):
 t=n+n/8;s=('vibgyor '*n)[:t];r=[s[~i:]+t*' 'for i in range(t)]
 for l in zip(*r+3*[' '+s]+r[::-1])[::-1]:print''.join(l)

Try it online!

Saved:

• -1 byte, thanks to Jonathan Frech