Usually, markings should be from the same colour family as the base coat, but the following ranges are also allowed.
Left: Free range for Lighter markings (ie. Underbelly, Nyala.) or dark markings with reversal.
Right: Free range for Darker markings (ie. Points, Stripe.)
Remember, nothing except eyeshine on a colonist should be true white (#fff) and nothing except the lines should be true black (#000). Line-art may be coloured but should be darker than what it's touching.