Colour theory is a collection of guidelines created for designers to follow. Colour helps communicate to the target audience by creating visually appealing colour schemes.
The Colour Wheel helps designers follow these guidelines. There are three versions of the colour wheel. One that contains just primary colours, another for secondary colours and the last for tertiary colours.
Colour harmony is the arrangements of colour in a design. If the colours in a design are visually pleasing, then it would be a harmonious design. If the colours in a design are chaotic or boring, then it is not harmonious. It all depends on how the audience engages in the design. If the colours are boring on a design then there will be no engagement from the audience, the same way if it was chaotic, the viewers brain would reject the design and not engage with it. When a design is harmonious, it creates a sense of balance, allowing the brain to understand it.