Make the nostrils larger with the dark colour. Draw a line down from the middle of the nose (between the nostrils) to the upper lip. You draw the whiskers with a fine brush.
If you do not want the colours to mix, be sure to wait for the first layer to dry before applying another colour over it or up against it.
Hint: Regularly change the water you rinse your brushes in and preferably work with two water bowls: one for light and one for dark shades.