You most certainly can make changes to the colours. I assume that you are referring to the "Try It", "Know It" banner when you click on the play.exe file?
To make the changes, navigate to the following within your UPK library:
1. System --> Publishing --> Styles --> <content> --> Player
2. Double click on Player, and you should see a number of files.
3. Navigate to the Skin.css file and open it.
4. The properties .navCellOff and .navCellOn defines what colours the buttons are. Change these values as required.
5. Save the change in the text editor.
6. Press the save button in UPK.
7. Publish topic to see changes.
8. Access play.exe and you will see the changes. I hope this is what you are looking for.
Also, have a look at the Content Development manual provided with UPK - spcifically the section "Customize Player Mode Buttons" on page 362 of the document.