I did not specify value above, even if it is closely related to color, because value is more general and represents how dark or light a design is. Value has a lot to do with mood too, only at a more profound level. Understanding colors will take you close to perfection, but knowing how value works will take you beyond this. Lighter designs offer a different impact and feeling than the dark ones and you need an expert eye to notice differences and decide which one is the best.
The space and how it is used is crucially important in design. Lately the “white space” (also called negative space) became used widely because it allows the human eye to read easier. For whoever is not familiar with the term “white space”, it does not mean precisely space filled with white, but every area of the design which is only filled with the background color. You can see several examples below to better understand the concept.
If there is a lot of negative space in your web design, it offers light and an open feeling. The lack of white space will turn your design into an old-fashioned, cluttered one. The space has also a lot to do with how the design is perceived by the human eye. Even if I said the color is maybe the most important element of a design, the space is definitely present in the top, because it is also very easy to notice by the untrained eye. It can turn a design to your advantage and get the best out of your layout.
Google is the best example on how the negative space can be maximized.