Ability to set individual field color

- [ ] I have searched the [issues](https://github.com/antvis/g2plot/issues) of this repository and believe that this is not a duplicate.

### What problem does this feature solve?
Displaying data with correct colour indicating severity of data. Red for critical, Yellow for Medium, blue for Low

### What does the proposed API look like?
color: [{fieldName:"Critical", color: "red"}]

The issue names no files, tests, or entry points; begin by locating the chart color configuration and its public API. Confirm how field names map to severity values, then verify that Critical, Medium, and Low data render with their requested colors.

Written by the indexing model from the issue text.