This is a small issue, and may not even be unintended, but it seemed inconsistent.
When I create two series with the same category field, they are the same color, as expected. For bubble charts, when I assign each of these series a different colorAxis each series is colored according to its own colorAxis. However, for line charts, both series seem to be colored according to whichever color axis was most recently assigned to them. The only way to assign to line chart series different color axes is two give them different category fields.
This is a small issue, and may not even be unintended, but it seemed inconsistent.
When I create two series with the same category field, they are the same color, as expected. For bubble charts, when I assign each of these series a different colorAxis each series is colored according to its own colorAxis. However, for line charts, both series seem to be colored according to whichever color axis was most recently assigned to them. The only way to assign to line chart series different color axes is two give them different category fields.
(see this stackoverflow post for further details + an example: http://stackoverflow.com/q/30174657/1560599)
Either of these behaviors seem reasonable, but it seems strange for them to differ by plot type.