Choose a lead colour and a supporting colour. The two do not need equal space: a small accent can have a strong effect when the contrast is clear.
Choose the relationship
Use neighbouring colours for a tonal result or a warm-cool pair for contrast. Decide which relationship fits the idea before selecting products.
Map the nails
Assign the lead to most nails and use the companion for a tip, line or one focal nail. Save the two product codes with the reference.