I know that I can do this with the Themekey module, which allows you to specify rules for which theme to use. You can make a rule based on the time of day and have two different themes, but then I will have to maintain two entirely separate themes. Since all I want to change is the colors, taking this approach would be a maintenance nightmare.
Pixture Reloaded uses the Color module to allow users to select the colors, so I don't understand how to easily override it.
One thing I noticed is that there are a number of pre-defined color schemes. For my purposes, it would be sufficient to have a way to automatically use one pre-defined scheme during the day, and another at night.
How can I do this without creating (and maintaining) a copy of my original theme?