Python 3, 72 70 bytes

lambda x:'#'+''.join(f"{round(int(x[i:i+2],16)/17):X}"for i in(1,3,5)) 

Try it online!

This is a port of Grzegorz Oledzkis original answer, which I helped him golfing down.

Two features of Python 3 help us save bytes:

• Floating point division by default
• Format string literals

Python 3, 72 70 68 bytes

lambda x:'#'+''.join(f"{(int(x[i:i+2],16)+8)//17:X}"for i in(1,3,5)) 

Try it online!

This is a port of Grzegorz Oledzkis original answer, which I helped him golfing down.

Two features of Python 3 help us save bytes:

• Floating point division by default
• Format string literals

-2 bytes thanx to Jonathan Allan