What are the functions of the Solar Plexus Chakra?
The solar plexus chakra is the energy center that helps you to keep the balance between confidence and conceit, between self-respect and narcissism, and between being bold and reckless. It also assists with your spiritual growth, your physical energy levels, and with you working through more complex emotional issues. It also rules the intuition and the ‘fight or flight’ response.
When this area is balanced, you’ll have no trouble determining whether it’s best to stand and fight, or whether it’s best to walk away. Your emotions are balanced. You’ll be able to form your own opinions without judging others in the process.
Another sign you have a healthy solar plexus chakra is if you get things done on time, without procrastinating or letting them stress you out. You appreciate your sense of self-worth, but at the same time you don’t undervalue others.
Where is the Solar Plexus Chakra?
The solar plexus chakra is found right behind your belly button, which is why you’ll see it called the navel chakra in some places. It’s in the lumbar region of your spine, around vertebrae L1 - L5
What Colors represent the Solar Plexus Chakra?
Yellow is the color of the solar plexus chakra. Yellow is confident, wise, and logical.
Yellow stones, jewelry, yellow foods, even just closing your eyes and meditating on the color yellow will help to restore balance to this area. 
Symptoms of an Overactive Solar Plexus
You may come across as arrogant when this chakra needs balancing. Oftentimes, overstimulation manifests as conceit or a lack of respect for others. You impose yourself on others and seek constant praise from outside sources.
You might also be perceived as a control freak or always needing the upper hand in situations or relationships. You’ll be frequently disingenuous with others, being who you think they want you to be, rather than staying true to yourself. You may have stress-related stomach problems.
Using Citrine and Yellow Jasper will help you to restore peace and harmony to an overactive Solar Plexus chakra. 
Symptoms of an Underactive Solar Plexus
When this energy center is underactive, you’ll find yourself feeling lethargic and your enthusiasm for life waning. You may become more reserved and struggle with low self-esteem. You fear rejection, it’s difficult to focus, and you might even be susceptible to digestive ailments.
In order to reactivate the Solar Plexus, you’ll have to find your way back to a place where you love and respect yourself. Bright yellows are the best colors to work with in this instance. Try getting outside and make sure you get plenty of yellow sunshine on your skin.
Look in your mirror and repeat positive affirmations until you recognixe your good qualities again. Rid your life of friendships and activities that make you feel unworthy, unimportant, or undervalued. 
Work with Tiger's Eye and Hematite to stimulate your Solar Plexus chakra.