They should be picked in the morning when the dew has dried on them. Roses need to be dried quickly to preserve their colour. They will shrink a little but will open up more when dried. Another way to dry just the petals is to carefully take off the petals and spread them out on a plate and then place them in the microwave on medium for 1 or 2 minutes. You can also lay the petals out on a clean cloth and place them in a warm area.
Reds go very deep and dark, pinks go to a deeper shade and yellows deepen only a little.