Depending on the color of Oni, the coloration of the Horns, Eyes, and Markings of an Oni are different. However, they all have the markings, horns, and fangs, consistently across the types of Onis.

Onis age similarly to humans until around age 25. After that, they retain a similar appearance until their mid to late hundreds, where they begin to become elderly. Onis are also born into their colorations, and can't change that. Silver Onis can't become Red or Blue, and this applies to Red and Blue, they can't become each other, nor can they become Silver. 

Onis do not tend to be lawful, more being chaotic and evil. The Oni prefers to do what they wish to do, however an Onis alignment is typically determined by its Subrace.

Silver Onis are often Good, being graceful and good natured, wishing to help those around them.

Red Onis are often Lawful or Chaotic, adhering by their own code to help humans, or simply being chaotic bundles of energy who prefer to be free of any restriction.

Blue Onis are often Evil, hating those who aren't other Blue Onis, preferring to be alone and isolated.

Due to their creation under the hands of the Goddess, Onis are knowledgeable about Religion. Young Onis, even Red and Blue, are taught about the Goddess who created them. Due to this, all Onis have an advantage on religion checks.

Due to their association with darkness and shadiness, Onis have developed Darkvision, often due to living in dark places for long periods of time. The Onis take pride in this ability, as it makes them more useful in all ways, for Red it allows them to help humans more, Blue lets them work deep into the night, and Silver it allows them to navigate to places to help even in the night.

Any member of the Oni race is born strong, with the strength of their race inherently in their blood. The Oni are strong and they utilize this in many ways. Along with +2 to their strength score, Onis have advantages on all Athletics Checks.

Onis speak two languages, common and infernal. Despite being created by a Goddess, the Onis are popular with demonic races, thus, they have learned to commune with these demonic entities. The common explains itself.

Onis are born strong and physically adept. Due to this, Onis are faster, with a 50 foot movement speed. 

However, despite being physically adept, Onis are really bad swimmers, often sinking instead of swimming. For this, they have a 20 foot swim speed.

Onis, along with aging somewhat similarly, grow somewhat similarly to humans. Onis grow until they're about 25, when they stop aging for a time, leading to Onis usually being somewhat taller than humans. In fact, the tallest recorded Oni was 7'9". However most Onis are about 5'11" to 6'7". Onis are classified as Medium Size.