Sit next to the student at the table. You should have the color tablets box near you, out of range of the child's focus area.
Lay out seven shades of any one color that the child is already familiar with. They should be mixed so that they do not make a gradient. Yellows, reds and blues are good to begin with because these will be the most familiar.
Ask the child to find the darkest shade. Do not begin by explaining the difference between dark and light. If necessary, you can answer when the child asks, but assume that he knows what you are asking for. If he selects the right color tablet, then praise him and place it on to the left slightly above the mixed up tablets. If he selects the wrong tablet, replace it, saying, "No, I want you to find the darkest color first." Allow him to try once more before explaining in any more detail.
Continue to line up the tablets from darkest to lightest. You can just keep asking the student to identify the darkest color remaining until there are none left. When you are done, you will have a gradient that starts with the darkest shade on the far left, then moves toward the lightest shade on the far right.
Encourage the student to repeat the exercise on her own. Once she begins working by herself, you should not correct her or check her work unless she requests that you do so. Montessori experts believe that colors cannot really be fully "taught" because you cannot teach someone to see. As a result, the child must self-correct in order to really understand the difference between dark and light shades.