Since your Core is one of the first ones you’ll need to either connect it to a WEP network or perform a DFU deep update before connecting to an open/WPA/WPA2 network.
But for the basics have a look here
http://docs.spark.io/troubleshooting/#troubleshooting-can-t-get-connected
I get solid magenta when using a factory firmware (without deep update) with correct credentials against my WPA2 network.