It’s one of the most noticeable design principles in floristry.
This particular vase arrangement is using a complimentary colour scheme. Complimentary colours are positioned on opposite ends of the colour wheel and provide the strongest amount of contrast when paired together.
Other popular colour schemes include: monochromatic (one single colour), triad (3 colours evenly separated on the colour wheel) and analogous (colours next to each other on the colour wheel).
Design principles and techniques are covered in full in our Complete Floristry Course.