UI Chart - Ensure consistency in mapping chart series to colours

### Description

When assigning colours to bars/lines on a chart, we assign the first message to the first colour, etc...

Sometimes though, users want a topic to be a specific colour, even though it is not known in which order they arrive. We should provide a way of defining colour:series mapping

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate

Start by tracing the UI Chart code that assigns colours to incoming series; the issue does not name a file, entry point, or test. Define how a user-supplied colour-to-series mapping should behave when series arrive in different orders, then verify that bars and lines consistently use the configured colours.

Written by the indexing model from the issue text.