The Ninjas hate cold weather and can be stubborn when it comes to starting.
In some cases starting fluid can be used ( you've got the right spot in your pic) to get them going, but should not become habitual. So having a properly tuned bike helps a lot (anti-freeze, plugs, oil & valve adjustments in particular.)
Always make sure your gas is fresh as possible. Winter gas blends tend to get the worst mpg, but are formulated for easier starting. The lowest octane is preferred.
Make sure you spark plugs are not fouled.
Sometimes changing the plug heat range can help with cold temp starting and running. You may have to remove the tank to properly access the plugs.
Always use full choke when starting. Your starting cycle and warm up time will depend on the temp. You should be able to pull off with the choke on to finish the warm up.
Ensure the idle speed is set correctly. This is set after the bike is fully warmed.
If the bike has or will be sitting for a while, you may want to add some Star Tron Fuel Treatment
to the tank to help keep the carbs clean during the riding year and storage as well.
Also getting the bike out of the cold can keep the bike 20-30 degrees warmer.