Colour fades for three reasons: water, heat and sun. You cannot avoid any of them, but you can soften all three.
Wait before the first wash
Give new colour 48 hours to settle before shampooing.
Turn the water down
Hot water lifts the cuticle and lets colour out. Lukewarm, with a cool rinse to finish.
Wash less, mask weekly
Two or three washes a week with a sulphate-free shampoo, and a repair mask on Sundays.
A gloss between colour appointments costs a third as much and buys you six more weeks.
Protect from heat and sun
Heat protectant every time, and a hat or scarf on the beach.