Spiritual Pressure

Spiritual Pressure (also referred as "Reiatsu") is the physical force/pressure that a person's Spiritual Power creates when released. Most Soul Reapers and Arrancars, and even Humans can manipulate the release of their spiritual pressure. The most powerful soul reapers that have a vast amount of Spiritual Energy, which they can emit in the form of Spiritual Pressure with devastating effects.

Spiritual Pressure (Reiatsu) VS Spiritual Power (Reiryoku)
The difference between spiritual pressure and spiritual energy (power) is simple: Reiryoku is the amount of energy a being has storedwithin their body and soul, whereas Reiatsu is the pressure that a person's spiritual energy (power) exerts. The difference is much like energy and power. Power being the amount of energy exerted per unit of time. Spiritual Power is potential while spiritual pressure is energy in use. In general, those with high levels of spirit energy will often have the highest levels of Spiritual Pressure also. A skilled warrior can overcome a person possessing greater Spirit Energy by possessing greater Spiritual Pressure.

Colors
Differents spirits have different colors of spiritual pressure, and it does not depend on the character's race or power: instead, the color is determined by the owner's personality.
 * Blue:
 * Light Blue:
 * Green:
 * Light Green:
 * Yellow:
 * Golden Yellow:
 * Red:
 * Pink:
 * Brown:
 * Black:
 * White:
 * Orange:
 * Purple:

Advanced Colors:

 * Black with Red Outline:
 * Black with Green Outline:
 * Black with Orange Outline:
 * Black with Purple Outline:
 * Light Green with Dark Blue Outline

Shape
Some particular characters have a shape or manifestation of their energy whenever they release enough of it. The manifestation are the one of the characters inner strength/energy.