how to specify colors

HI sachiko,

>>Is it possible to specify a color for a particular value in the graph?

To be honest, I'm not very familiar with this feature, but I think it is impossible to specify a color for particular value.

For similar requirement, I'd like to recommend you take a look at conditional formatting.(notice: this feature only available on some specific visuals)

Conditional Formatting in Power BI

I'd like to suggest you take a look at following article which mentions custom theme json and properties:

Use Report Themes in Power BI Desktop (Preview)

Regards,

Xiaoxin Sheng