It depends on what colour you want and if you are trying to match existing mortar. It also depends on the type of brickwork. For example older, soft bricks should normally be re-pointed with lime mortar. Some mixes use a 50/50 white/grey OPC to get a different colour.
For newer brickwork a 1/1/6 lime/cement/sand mix is often used. Stronger mixes are sometimes used in exposed areas near the coast, and with very hard engineering bricks.
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