The syntax highlighting of hex files is bad, as shown in What PIC processor was this HEX-file meant for?:
Can something be done about that?
Depending on the tags you use, different code syntax highlighting will be used. You selected the assembly tag, which has a code syntax style default. It's not exactly assembly because there isn't a code format for that available.
I wouldn't expect a "proper" syntax highlighting for your code as that is more of a file format (Intel Hex) rather than a code language (C, ASM, Python, etc...). The syntax highlighting is done using Google Prettify (see editing help for more info).