So, making an Op4 mod is straightforward, but what you're aiming to do is complicated. The game dlls hold things like enemy behavior, weapon behavior, and flashlight/night vision stuff. It sounds like you want some elements from HL and some from Op4, and mixing them in this way is probably going to require you to code your own dlls for your hybrid mod.
If you can live with the Op4 night vision and an Op4 style HUD, you should be able to recolor the HUD easily enough.
edit: Sorry, was tired when I wrote that. What it should say is that if you're using Op4 you can't recolor the HUD but you can change how most of it looks since that is set in sprite files.