Hi Clara,
Chances are, you don't need to modify any PHP at all -- the changes you'll need to make are to the style.css file (or whichever is the main stylesheet for the theme).
If you don't feel like modifying the specific link code (of which there may be many - couldn't tell you without knowing which theme or seeing the code), then you can paste what I'm about to offer at the very bottom of the file; however, you should keep in mind that it will override
all text links within the document.
Code:
a {color: #XXXXXX !important;}
Now for the explanation. In the above, you need to replace XXXXXX with the hexadecimal value for the color you want (example: white=FFFFFF, black=000000, red=FF0000 and so forth). This basically changes the text color for the links. The !important says that no matter what any other rules in the stylesheet says, always use that text color for your links.
Frankly, that may be a little too strict for the theme and it may cause the design to look off, for lack of a better word. If you care to tell us which theme (with a link, please and thank you) or, even better -
post a link to the stylesheet (that way, I won't need to download/unzip/open any files), and mention where the links are you want the color changed (i.e. content, sidebar, header, footer, etc.) and the color you want, I could give you something more specific.
~ Teli