Many colours mean many types of colour, and much colour means a large quantity of colour, is it right?
Colourful - Having much or varied colour
Many colours mean many types of colour, and much colour means a large quantity of colour, is it right?
Colourful - Having much or varied colour
You would not normally use "much colour". Having "much colour" would mean it is colourful. A bright-red coat might be described as colourful, even though it only has one colour.
It is acceptable to use this in the negative: "The coat doesn't have much colour" It would mean it is mostly black, white or grey. It could mean that the colours are pale.
Dictionary definitions are often not examples of natural English. This is because the idiomatic way to describe something is to say "It is colourful" But the definition of "colourful" has to find an alternative word or phrase that means the same. That phrase might have to be non-idiomatic.