Is there anyway to change/add more highlighter colors in Preview? I need more colors aside from those already provided in the app.

Unlike Preview, the Adobe Reader application allows you to change PDF highlight (comment) text colors via a standard Apple color chooser. Click Tools on the Reader Toolbar, and then Comment. The second button in the Comment Annotation section is highlight text, and defaults to canary yellow.

Highlight the document text, and then two-finger tap/control-right-click that highlighted text. On the contextual menu, you will see Properties…. The properties panel has a color well set to yellow. Click this color well, and a standard Apple color chooser will appear that allows you more creative control over your highlight color. When you click a color in the color chooser, it is not automatically updated in the color well, which remains dark grey. When you quit the color chooser, the selected color is updated in this color well and in your document. The next text that you select to highlight will receive a default yellow highlight.

Actually, the better tool for the job is the free Skim PDF Reader and Note taker application. You right-click the PDF document, and choose open with Skim. In the Tool Mode array of buttons, click the downarrow incorporated into the H button. On this submenu, ensure that Highlight is checked.

You won’t be presented with an i-beam cursor, but you can then click and drag right to highlight passages of text, or single click on individual words. Default color is canary. If you right-click on this color text, the sub-menu will offer Current Note Color… and clicking this menu item will launch the Apple color picker. A single click on a different color immediately changes the highlighted text color in the PDF.

There are five highlighter colours available, you can't add other colours.

Alternatively use the line and box tools which have millions of colours available