Hi Panther,
Since you prefer using Humbrol enamel paints like me, you might find this conversion chart handy -
http://smmlonline.com/articles/royal_navy_paints.html
You will see that the Admiralty paint codes go something like AP507A, AP507B etc etc.
If you go to the HMS Hood.com website, you will see that they give Admiralty paint codes for Hood during the various years of her career.
The Trumpeter kit allows you to model her as she was when sunk in 1941, so you need to go to HMS Hood.com and read through the paint guide for 1941. Here, they give Hood as being painted Admiralty Royal Navy colour AP507B.
If you look at the smmlonline RN paint conversion charts, you will see that Admiralty colour AP507B is Humbrol colour H145 Medium Grey.
Hope this helps.
BTW - The underside/below waterline should be Humbrol brick Red.