terminal: improve color matching, diff readability

[tstromberg]

Fixed 3 color inconsistencies:

• The green used by "diff" to show additions matched the green used to denote low-priority behaviors
• The "orange dot" used by "HIGH" did not match the red text, and was hard to tell apart from the "yellow dot" (MEDIUM).
• The "red dot" used by "CRITICAL" did not match the magenta text

In diff mode, this also adds a 3rd "-" or "+" to more clearly show adds/removes