Flashing red and blue is a common indicator that the phone forgot pairing information, and it is in pairing mode.
On the iphone, go to Settings --> Bluetooth
If the headphones are listed, click the "i", and select "Forget the device".
Now, while the headphones are flashing red and blue, on the iPhone side, you should see your headphones listed again under "Other Devices".
Simply tap the name of the headphones to pair it again.