Hi Afham -
You first have to go into ow_includes/config.php
Scroll to the bottom of config
Change dev mode from "false" to true.
Re-upload config file.
Now you may change the color # in the base.css file in the folder of your current theme.
Don't forget to change config file back to "false" for dev mode when you are done.
If you are unsure about doing any of this - dont break your site - search this forum for these things I've mentioned and you will find more information.