Highlighter color should be changable as with text or pen drawings:
Select the highlight (Select Rectangle Tool)
Change the color using the color buttons (does not work)
"It's a feature" -- because when you select both pen drawings and highlights and then press the color button, you really don't expect the highlights to also change to that color. (Believe me, it was the behavior I first coded in and I couldn't take it).
I assume one possible change to the behavior would be: if the select rectangle contains only highlights, no text or pen strokes, then the color button changes the highlights' colors. Or: if the currently selected tool is the highlighter and the select rectangle contains some highlights then pressing the color button changes the highlights' colors (but not those of any text/pen items) ?