The colors of clashing items is determined by the status of the clash, which you set in the Results tab:
Red: New
Orange: Active
Blue: Reviewed
Green: Approved
Yellow: Resolved
Rather than changing the statuses for the sake of changing colors, if the clashing items are not standing out, you should try marking a combination of the checkboxes in the Display panel in the results tab, such as Auto Reveal, Dim Other, Transparent Dimming, or Hide Other, and your clashes item should then stand out from the rest of the model.
Information on statuses and the Display options can be found in Navisworks by selecting Help > Help Topics > Contents > Locate and Manage Interferences > Overview of Clash Detective Tools > Clash Detective Window > Results Tab.
