If you want to change specific bullet you can do the following;
Select the bullet point(s) to change, then right click of the selected point(s) and select Bullets and Numbering... from the menu
From the Bullets and Numbering window, select the Graphic tab, and choose a colour bullet. I've tried looking through the menu, but couldn't find another of changing the bullet colour other than using a graphic.
Further still you could modify the style for the bullet points, perhaps allowing you to switch between styles much easier. Click F11 to bring up the Style and Formatting window, then select the List Styles icon, and modify or create any style you want there. This blog post might be of interest for further reading, World Label Blog - 9 Tricks for Using List Styles in LibreOffice/OpenOffice.org