<font color="white">Text that's meant to be hidden.</font>
the color option will be removed from your code. You will end up having
<font>Text that's meant to be hidden.</font>
Other font colors that are darker are supported though.
I think this is done specifically to subvert some people that are hiding keywords who do it so that their item comes up when you search for something else.
This feature however is very easy to bypass. You can still use the HTML FONT tag but add a size option. For example, the following code could be used to hide text on your Craigslist post:
<font color="lavenderblush" size="1px">Text that's meant
to be hidden.</font>