Cocktail on an Indian wedding invitation usually means an evening event, a reception or a party, sitting below the main ceremony in formality and well above ordinary office wear. For men that means dressed well without the full ceremonial kit: a good silk bandhgala, a Nehru jacket over a fine kurta, or a Western suit in a rich colour. The invitation is asking you to look sharp, not to compete with the couple for attention.
The garments that actually work at a cocktail event
Three options cover almost everything. A raw silk or dupion bandhgala in a jewel tone or a rich neutral. A Nehru jacket in textured silk or a structured cotton, over a matching or complementary kurta. Or a Western suit in a strong colour, deep navy, burgundy, forest green, with a good shirt. All three land in the right place: clearly dressed up, not dressed for the mandap.
What doesn't work: a heavily embroidered sherwani or achkan, which belongs to the ceremony and looks like you got the date wrong; a plain cotton kurta with nothing on it, which reads as if you came straight from home; and dress codes imported from the wrong tradition, such as a morning coat at a garden cocktail party. Match the register to the event and you'll be fine.
Colour: standing out without trying too hard
Cocktail events give you more colour licence than the ceremony does. A midnight navy bandhgala, a burgundy silk kurta with a bandi over it, an emerald Nehru jacket: all of these say something about you and still photograph well under evening light, which is where most of these events live.
Leave white and ivory to the couple. Skip very pale colours, which go flat and grey under indoor evening light, and skip anything that looks like holiday wear, because a big floral print reads as resort rather than reception. A solid, rich colour in good cloth is the choice that keeps working across the widest range of these evenings.