You know now not to use the technique since SEs can read your CSS and will ban a site for hiding content from users, but not for robots.

The above notwithstanding, you must also pay close attention to the contrast between foreground and background colors. If the colors are too close in hue and density it could be construed as 'an attempt to hide content' from human readers. It would raise flags, possibly requiring human checks, which would definitely slow down indexing.

The general message should be clear: Don't hide text from readers that is intended for robots. This goes for comment spam, too (keywords stuffed into html comments). Contrary to what some may believe, body text is important, and should be rich and topic oriented for easy indexing.