The gums should be a healthy pink color. When you press them with your finger, they should lighten up and then go back to normal within seconds.
You worry when the tongue turns white or grey. This indicates that there is decreased bloodflow. This can be caused by stroke, heart conditions, or even heat related issues.
I would definitely go ahead and give the vet a call now to see if they want you to bring her in.
Let me know if you want to discuss this more.