Colour gamut is the range of colours a particular standard, source or display can represent. Correct presentation requires the image's colour information to be interpreted appropriately throughout the playback chain. A wider display gamut can be useful for suitable material, but it does not mean that every programme should be expanded to maximum saturation.
A practical example
A television can display an ordinary SDR programme accurately within the intended colour arrangement, then use a wider supported range for suitable HDR material. Applying a vivid mode to both sources can change their appearance without making either more faithful.
A common misunderstanding
More saturated colour is not necessarily more accurate colour. Avoid using a wide-gamut setting as a universal fix for a picture that is actually suffering from incorrect source interpretation or signal levels.
Related terms
Stable technical definition. Editorial approach