This certainly can be done. Just change the condition from "Value is" to "Formula is" and then you can access the full calculation engine.
In your case, then you would specify the conditional formatting for the "January" field.
Let me know if you need further help with getting this set up.
TS_Oz, FileMaker Inc.
"Can I use conditional formatting based on the value of another field in the same record?"
As TS_Oz says, yes, you can. Not only can you reference a field in the same record, but you can have conditional formatting reference a field in a record in a related table as a condition for your formatting. This is very useful.