You need to state the name and full version number of the story format you are using, as answers can be different for each one. I will assume that when you stated you are using Harlowe that you really meant v1.2.4 which is the default version, and not v2.0.1
You can use CSS based on the names you are giving your named hooks to style each of them.
First a little background, when you use a named hook like so:
... the HTML generated for it looks like the following:
<tw-hook name="Guy" class="" style="display: inline-block;">"Hjojojoiioijoojo."</tw-hook>
... and you can use the type of the HTML element (tw-hook) and the value of it's name property (name="Guy") to create a CSS sector which will allow you to style all named hooks with the same hook name.
Place the following CSS within your story's Story Stylesheet area, it will change the foreground text colour to green.
... notice the usage of the HTML element's type tw-hook and the name "Guy" of the named hook.
For more information on what CSS you can use to style the HTML element I suggest using one of the on-line tutorials like the CSS Tutorial at W3 Schools, or just ask more questions here.