Has it been hot recently? Some colours (eg himalayan, shaded and tort) are temperature sensitive. The points (nose, ears, tail and feet) are darker because they are naturally colder. When the ambient temperature is high, new hair growth comes in lighter so the coloured areas shrink and/or lighten. When the ambient temperature is low, new growth comes in darker and the coloured areas grow and/or darken. For the same reason, these colours tend to be light at birth and then darken (because it is very warm in the womb and cooler in the nest).