As the subject says.

I immediately put vinegar and bicarbonate of soda on it, but there's still a stain there.

Make a paste of dish soap and oxygen cleaner, work it into the stain with a brush, let it sit for 20 minutes, then rinse. That has worked well for me. You may also want to ask the supplier as they know more specifics about what pigment they used.

Probably no help, but once you've worked some detergent into the carpet, a wet vacuum cleaner (VAX or similar) is absolutely invaluable for getting stains out.