The chart colours seem to be preset and random.
Is there a way to control the colour?
Other than the preset colour schemes, no.
For more advanced charting, I’d recommend taking a look at quickchart.io
Also, see the following topic:
Looks pretty good, except now have to move and format the data for the API…
It’s not that difficult, and you can make some very nice looking charts.
There are lots of examples in that topic that I linked to.
quickchart also has a sandbox editor that you can use to get the basic structure of your chart, which you can then essentially just plug into a Glide URL column (with a few extra parameters and substitutions).