There is a good step by step guide in the GW
book “How to Paint Citadel Miniatures”. The miniature was an ork. I believe it is based with scorched brown (or equivalent) then bestial brown, followed by vomit brown, bleached bone, and finally skull white. The key for me was building up the thinned down layers that formed the “tentacles” which work their way up the horn (tip of the horn being the lightest color). You could also use black and gray like others have suggested
. Took a few tries but worked out real well. Good luck.