This chakra is often confused with the navel chakra, and most writers refer to only a single chakra, located either at the navel or the solar plexus. However, these two centres are quite distinct

The Solar Plexus Chakra is located midway between the navel and the base of the sternum. It supports the spleen, pancreas, stomach, and liver [Chia, p.243]. Each of these vital organs in addition is associated with its own chakra, just as the heart, lungs, and kidneys are.

The Solar Plexus Centre is one of the main power chakras of the individual. It is associated with the functioning of the aura or psychic energy field, and with Etheric and Astral plane sensitivity [Chia, pp.245-6]. Through this centre, which seems to correspond in part to the "Spleen Centre" of Theosophy, the various spiritual energies enter for distribution to various parts of the body [Leadbeater; Paulson, p.84]. The Solar Plexus Chakra is also the centre of etheric-psychic intuition: a vague or non-specific, sensual sense of knowing; a vague sense of size, shape, and intent of being [Brennan, p.173]

This chakra is sometimes described as yellow in colour, although it would be more accurate to consider it as orange or yellow-orange.  In the traditional "rainbow chakra" theory, the Solar Plexus chakra is generally described as yellow in colour. Because yellow is also the colour of the Lower Mental Body and the Higher Spiritual Intuition, some writers refer to this as a "mental chakra" or associate it with rational-intellectual faculties. This is quite incorrect. The Solar Plexus Chakra - like the Heart Chakra above it and the Navel Chakra below it - is a purely Emotional Centre.

Correspondences:

Element: Traditional Chinese (associated organs: Liver & Spleen) - Wood, Earth

Colours: Traditional Chinese (associated organs) - Blue/Green, Yellow; Hills - Yellow.

Associated Psychological states:

Open and Balanced: You have a deeply fulfilling emotional life [Brennan, p.75]. You take an expansive, outgoing, and sociable attitude to the world [Beasley, p.217]. You are able to feel the sweetness of life [Paulson p.84], and have a sense of belonging and fitting in perfectly in the universe. You feel daring and confident and able to take risks [Chia, p.246]. Hence this is the chakra associated with the leader, the explorer, and the charismatic extrovertive individual.

Overactive: You are ruled by ambition, pride, fear, love of fame, desires and passions, and likes and dislikes [Aurobindo, p.334]; you are overly concerned with their place in life and their relation with others; you may crave the company of other people and be unable to cope or amuse yourself on your own.

Malfunctioning: You are easily overwhelmed by emotions or caught up by astral forces [Brennan, p.75], and may suffer from delusions and schizophrenia.

Blocked: You tend to block your emotions, or feel only superficial emotions [Brennan, p.75]; or you may feel only negative emotions such as anxiety, panic, or worry [Chia, p.246]. You may be closed to spiritual energies and unable to experience the sweetness and joy of life. You have a fear of change and are incapable of opening to greater things in life [Paulson p.84].

Qabalistically, this centre might be said to correspond to the upper part of sefirah Netzah ("Venus"); the counterpole of Hod on the rear line of the body.  Whereas Hod represents the controlled, "Apollonian" approach to life, Netzah represents the spontaneous "Dionysian" outlook [Dion Fortune The Mystical Qabalah].

The next chakra after the Solar Plexus is the Navel Centre, which of course brings us back to our starting point, and completes the survey of chakra psychological types.

major and minor Chakras | Notes on Psychological terms

rear chakras ascending		front chakras descending
Crown
Back of Head		Upper Forehead
		Middle Forehead Lower Forehead Brow
Mouth of God		Talu/Lalana
Back of Neck		Throat
Base of Neck		Thymic
Shoulder Blades		Heart
Diaphragmatic		Solar Plexus
Door of Life		Navel / Sea of Ch'i
Sacral		Pubic
Coccygeal
Perineum