Your navigation buttons are a repetitive element. Colors, style, illustrations, format, layout typography, can all be part of the repetition that unifies the entire site. Contrast might be type that is bolder, bigger, or a very different style. It might be different colors, graphic signposts, or a spatial arrangement. To be effective contrast must be strong.
Contrast defines what is important.
If two elements such as type, graphics, color, texture, etc. are not the same, make them very different, don't make them almost the same. There are times when you don't want contrast - when you want to present continuous text as in an article. Links also provide contrast. If you want people to read an entire piece, let it be bland and uninterrupted.