Found a clunky work around. Create the report via the GUI, set permissions, schedule it, enable embedding, etc. Then go to $SPLUNK_HOME/etc/apps/search/local/savedsearches.conf and find the stanza for the report you just made. In there you can add lines the same way you would edit a dashboard element via XML:
colors of your bars, lines, etc: display.visualizations.charting.seriesColors = ["0x5c2fb6"]
color of the deliminations: display.visualizations.charting.foregroundColor = #000000
color of the axis and labels: display.visualizations.charting.fontColor = #000000
color of the background: display.visualizations.charting.backgroundColor = #ffffff
But, as far as I can tell, this file is not read on the fly, and you need to restart Splunk for any color changes to take effect.
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