there will not be a touch up paint that can match the finish and the coverage. To avoid buying new shifter/levers, take off the rubber, mask off everything tightly, sand BOTH levers to the same finish and consistency and have them both airbrushed with the same primer/paint. Anything else will show that scratch.
If you do it right, you can do it cheaply.
that said...damn dude. It's only ultegra.
You should see my shifters. They look like they've been through a war, but they still shift and brake. You just can't have nice parts in NYC.