Welcome to the Serif Affinity forums.
The noise in those default styles is intentional, as far as I know. You can edit the colors once you've applied them to an object and adjust the noise levels for that object. For an object with a solid color use the Color panel and clock on the small circle next to Opacity to change it to Noise. If the object has a gradient I think you'll need to do that adjustment using the Fill or Gradient Tool, depending on the application you're using.
