Is it possible to do conditional formatting in Numbers for iOS? I'd just like to change the text or background colour depending on the cell's contents.
iOS Numbers does not support conditional formatting, and it is removed from the document upon import.
According to http://support.apple.com/kb/HT4648 the following changes are made on iOS (my emphasis):
3D charts are converted to 2D charts. Chart data is retained and editable.
Conditional formatting is removed from imported spreadsheets. Cell content is retained.
Table Categories are removed. Tables remain organized and category names are retained as static text.
Filters are removed.
The most recent version of numbers on Mac supports conditional formating rules. The current version of iOS numbers does support conditional formating, but only shows UI for simple rules (like turning a number red when it's negative)
To access these, select some cells, then tap the format button (paintbrush), tap format, and then the more info/(i) button next to one of the options. From there you can chose some simple preset options.
If you want anything more complex, use numbers for Mac.