What is the extension of the file you're opening? By any chance is it a header file with no file extension? If you open a second file then switch back to the first, does the color coding show up?
A good quick check when this happens is to go to the "Document" menu and look for the "Select Mode" menu command. If the command looks like "Select Mode (C/C++)...", then SlickEdit is correctly determining the file type, and you should see color coding right away. So this is either a refresh problem, or there is something in the file that is tripping up the parser.
If the menu command looks like "Select Mode (Plain Text)...", then SlickEdit was not able to determine a language mode from the file extension or make a guess from the content of the first few lines in the file.
If you want to test this out with another language, you can go to the Project > All Workspaces > Sample Workspaces, and select the sample Java project.