To stop people accidentally booking a colour without a cut & blowdry, it is mandatory that any service with development time must have a second service booked after it in order to proceed.
I.e unless you tick a finishing service, the 'Next' button won't go purple
If you have services with development time that don’t require a second service, you may wish to create a new service that is free, such as “Rough Dry” or “Complementary Head Massage” so that your clients can still proceed with their booking.
When you create this service, be sure to go to Online Booking Settings afterwards and make sure the service is “ticked on” for all bookable staff.
For more information about setting up and customising Online Booking click here