In graphic design, variety and unity are essentially opposites that play off one another before the eyes. The most essential thing to do with these principles is to create a ballance within their visual dance. Whilst unity maintains a feeling of security and wholeness, variety provides interest and the desire for more engagement.
Unity creates wholeness. Unity is the overiding principle to which all other elements and principles are bound. Unity is the control of variety. Unity is usually achieved when the parts complement each other by having something in common. Unity is achieved by use of the same colour, or different tints of it, or by using a similar graphic style, thickness of line, shape etc. within the design.
Compare the two diagrams. The first is a plain grid with a high degree of unity: the shapes are all the same, the same size, colour & texture. In the second diagram again, the shapes are all squares but they vary in size and shade. This creates interest as wellas unity.
Definition of unity in variety: a principle that aesthetic value or beauty in art depends on the fusion of various elements into an organic whole which produces a single impression.
The quote above emphasises the need for your design to have a coherent wholeness despite offering choices to manouver in interesting and unexplored ways. Both Comfort in familiarlarity and the excitment of branching out anew.
Variety can add interest and break the monotony of simple repetitions. Using a range of different qualities or instances of a design element to create a desired visual effect - e.g., a variety of shapes, colors, etc.
Be careful, the mind may find it an effort to explore the variety in your design; it can be over-fussy or there may be just one or two extra design elements that aren't needed. What is important is to create a ballance between the two principles. Remember to provide an over-arching unified element or style to permiate your design.
Unity creates solidarity, conformity, comfort and familiarlarity; so whilst the mind enjoys this security and comfort - it also craves the interest and excitment of the new. So we move between UI choices (which appear safe and familiar) but they may take us to new content and interfaces. ❧