How to set symbol color from an attribute or expression?

[MattWilkie1]

In ArcGIS Online how do I set symbol colour from an attribute column? (or an Arcade expression)

The features are broken into classes by a text Type attribute and styled with marker symbols. In my table I have a column of html/css hex colour values to read from. I want each symbol to be coloured according to that column.

I'm trying to replicate what it looks like in Pro using Attribute-driven color in symbology: