Add docs for prefixColors changes

README.md

@@ -250,6 +250,9 @@ concurrently can be used programmatically by using the API documented below:
     - `prefix`: the prefix type to use when logging processes output.
       Possible values: `index`, `pid`, `time`, `command`, `name`, `none`, or a template (eg `[{time} process: {pid}]`).
       Default: the name of the process, or its index if no name is set.
+    - `prefixColors`: a list of colors as supported by [chalk](https://www.npmjs.com/package/chalk).
+      If concurrently would run more commands than there are colors, the last color is repeated.
+      Prefix colors specified per-command take precedence over this list.
     - `prefixLength`: how many characters to show when prefixing with `command`. Default: `10`
     - `raw`: whether raw mode should be used, meaning strictly process output will
     be logged, without any prefixes, colouring or extra stuff.