There are radio buttons for Decimal and Hexadecimal. If you have Decimal selected, then I'm not surprised that you're getting this error!
Also, if a value should be '0' (zero), don't just clear the field, but make sure you actually put the '0' there. That's also a mistake commonly made.
The original value is 0x0001. When I change it to #FFFF, per your instructions, SimPe gives me the message "Property value is not valid".