This is just a theoretical page depicting the origins of the symbols used by the characters in canon. I'm sure that for some of the species who have a very in depth and already figured out background (Like the Serrangios and the Martedians), this will kind of be considered non-canon, but I'll just put my ideas forth anyway.
Symbols are assigned in accordance to their blood. The symbol's colour is based on the blood colour of the troll with that symbol. Symbols also relate to their blood caste system.
Symbols are assigned to all the descendants of a certain bloodline, based on the legendary item of that bloodline's ancestor. Mates on this planet will duel in order to decide which of their bloodline symbols will be bestowed to their children. Symbol colour is based on the colour of the legendary item as well.
Splationians are born with a distinctive birthmark on their wrists which is represented as their symbol. The colour of these birthmarks is a mix of the colours of the parent's birthmarks. The cyan colour is considered the median of the birthmark colours, and hails traits of magical powers and physical abilities in balance with each other, as well as having the most longevity of all the colours on this scale. Colours closer to the sides of orange and red on this scale hail traits of stronger magical powers, weaker physical abilities, and less longevity as the colours get closer to red. Colours closer to purple and violet hail traits of weaker or no magic powers, greater physical abilities, and less longevity as the colours get closer to violet (the scale of longevity is equal on both sides and distance from the colour cyan. Touch of the birthmarks will allow the Splationian to unlock and utilize their magical powers or physical abilities.
The Yoshies symbols are based on one's social ranking (and all that) in the society (royalty would have a crown symbol of sorts, merchants would have currency-based symbols, scribes and scholars would have writing/book based symbols, etc.) The symbols colour is the same as the eye color of the Yoshi who has the symbol.
Symbol colour is based on the eye colour of the ruler dark matter. As drone dark matters (Whatever it's classified, since DMS said that when Rev killed the yoshies and the other planet that he was a drone and all that.), the drones share the symbol of the ruler dark matter that created them, and when they become fully-conscious adults, they take on a symbol that represents an item that they used in the pre-consciousness to prove themselves as a warrior. If a dark matter is to betray or rebel against their ruler dark matter, then they are bestowed with the Traitor's Symbol and exiled.
Equastrians are granted with a symbol representative of the aspect of magic in which they have power and control. Several symbols are used by many different Equestrians, so many variations of such have been made to those symbols to differentiate between them, which includes involving the colour of their hair, skin, or eyes.
Binarellers are granted a symbol that is based on an item that they have utilized to prove themselves in their respective fields. The colour of the symbol is based on the chosen colour of their tribe's leader.
The symbol's colour is assigned to a Serrangio's squadron. The symbol otherwise is based on something that they would bring to the group or a weapon or other sort of item that they specialize in use of.