If you have a calc that already sets "Low", "High", etc., then just drag that calc onto COLORS. Tableau will pick the colors for you, but you can set your colors however you want.
A risk can have different evaluations for Impact and Vulnerability. It does not have to be the same. For instance, risk 1 can be Low for Impact, but can be High for Vulnerability, that means I need to have two separate columns, one in order to show the evaluation of the Risk's Impact (in this case color green) and another one, just beside the first one, in order to show the evaluation of Vulnerability (in this case color orange).
When I put two dimensions onto COLORS Tableau automatically join these fields and displays them in one column with random colors.
I'm confused. Your original question asked to color by Risk.
What do you also want to happen with Vulnerability? Do you want 4 colors? Or 8?
Maybe a workbook would help be understand what you are needing.