I've done a bit of spraying in the garage over the years restoring several classics in the process. 1 Ltr of mixed paint sounds about spot on for your bumper. 1 Ltr of paint for you to mix yourself (If cellulose) will give you approx 2 ltrs of mixed paint, so that would be way too much.
If it was me, I'd go down to your local automotive paint supplier and ask them to mix you two 500ml cans. You may not need all of it, but you'll have a bit spare for any future scrapes. If you take your time you should get an excellent finish. With metallics you must make sure you shake the can really well to get an even distribution of metal particles, and only spray on one plane or it could end up a little uneven. And as is key with all painting, do your preparation as it's probably 75% of the whole job. Two cans should be no more than £35.
Don't buy it from Halfords, it's mixed by people who will be flogging you a new bike one minute and giving you the full spec of a Garmin sat-nav the next. It's a specialist job to get it right as it relies on the correct amount of each pigment being weighed into the mix.
I hope this helps
Steve
