Colour attracts our attention more than greyscale shades. Colour can highlight something.
The only difference between greyscale shades is brightness. Colour adds hue as a dimension you can contrast. Colour can show that two things are separate.
Because we can recognise the difference between colours, colour can show that types of things are similar to each other.
Colour can communicate something about an interface. For example, you might colour all interactive elements orange. I don’t know how different this is from classification.
Groups of people associate colour with practical meaning. Red means stop. Colour can trigger these practical associations.
Groups of people associate colour with emotional meaning. Red means danger. Colour can trigger these emotional associations.
A colourful interface is more interesting than a greyscale interface. Colour can make something look better.
We don’t live in a greyscale world, so colour makes us feel more comfortable. Colour can make something feel more welcoming.