Get the fill color depending on age.

        Args:
            age (int): The age of the branch/es

        Returns:
            tuple: (r, g, b)

def _get_color(self, age):
        """Get the fill color depending on age.

        Args:
            age (int): The age of the branch/es

        Returns:
            tuple: (r, g, b)
        """
        if age == self.tree.age:
            return self.leaf_color
        color = self.stem_color
        tree = self.tree

        if len(color) == 3:
            return color

        diff = [color[i+3]-color[i] for i in range(3)]
        per_age = [diff[i]/(tree.age-1) for i in range(3)]

        return tuple([int(color[i]+per_age[i]*age) for i in range(3)])