Do not use explicit colours

[piefel]

The syntax file contains a lot of highlight definitions mapping to fixed colours. That’s not how Vim syntax files are supposed to do it.

[elextr]

It would be more useful if you provided some more information, a reference to the method you claim should be used, or better yet a pull request with a fix.

[piefel]

The correct method is already used for some definitions – linking to predefined groups:

highlight link asciidocCallout Label
highlight link asciidocEntityRef Special

Although, nowadays syntax files seem to do that with hi def link instead. The Vim syntax states which highlighting groups are standard, but this is a very short list. Instead, the standard colour files, such as slate.vim or zellner.vim, define a number of hightlighting groups that are a de-facto standard. Linking to them would be safest.