Visual perception of colour and size is a complex and incompletely understood phenomenon. While we understand how colours are formed and how the brain interprets images, there is much more to looking at an object; there is a complex interplay between its own dimensions and its surroundings. This includes the differences in colour between the object and its background, which can make objects appear more striking or distant.
Small objects of the same size that are in colour or shape order take on a grouped appearance and patterns that are visually different to the same objects arranged in different colour/shape orders. Colour and line placement are often used to make a 2D picture seem 3D, such as in a painting, with lighter or darker objects towards the back, depending on lighting. The fashion industry has made use of this theory too; dark colours with vertical stripes are said to make you appear slimmer than bright colours with horizontal stripes.
Answered by Aneel Bhangu, How It Works contributor.