When a gene colour rule states it has to be natural, it may mean a couple of different things!
It can be any of the following: Any natiural tones, ane colour from the base coat swatch (excluding malenism and leucism unless those genes are prsent), or a longter/darker shade of the coat itself. The hue, however, is not allowed to change.
Below, you can see what we consider natural tones, as well as a cisual on what you may change when it comes to different shades of the base coat.
Natural Tones
any of these colours count as natural:
Black
White
Cream
Tan
Brown
Grey
You may use any colour on the base coats palette, Not including in the Leucistic and Melanistic ranges, they may only be used with those genes. 
You may use shades that are Lighter or Darker than the base coat as well.
Your Hue value should not change when doing this. 