The rat fancy has been around for a long time now, and rats have been shown, formally and informally, for a lot of the time. Before the mass popularity of the internet, it was almost impossible to describe, over text, what color your rat was. There were blanket statements of color and tone that led to many assumptions, and an incorrect grasp on genetics led to still more errors.

Many of the colors we now standardized were mistaken for, or called, by other names originally. Names like Sand, Opal, Blonde, Cream and Powder all were names for colors now given other names. Some of them are considered "extinct" rather than just re-named, they are claimed to genetically no longer exist. There's no way to tell, without a time machine, how many of those colors were actually something that we now recognize under another name.

In order to establish a genuinely "new" color, it has to be proven. That means, the color has to be:

1) Recognized as a new color and not a dilute or mutation of a familiar one.

2) Bred against familiar colors to be sure it is in fact non-allelic to them

3) Inbred until it can be ascertained that it does in fact breed true and that other recessives that might be making the color look new are cleaned out.

This is a massive undertaking and would take years of dedicated work for a breeder and many generations. The problem with many of the "new" colors that are claimed is that the time is not taken to clean the other colors out of the line. If you have a New Color rat who looks very much like a Russian Blue, but when bred to a Russian Blue they produce Blacks... you know for certain it is not a Russian Blue. However, if the New Color rat is carrying Russian Blue, then it becomes much more of a challenge to determine exactly what it is. The more dilutes and recessives a rat is carrying, the harder it is to determine exactly what its true color and genetics are.

Most rat show clubs have some method by which new and provisional colors and standards are added to their list of showable colors. Those methods vary by club; it is best to do research on that club and find out how they do things before attempting to put forth a "new" color. Research first is always a good idea - you could find that the "tri color" rat you thought you had is simply a black rat going through a state of constant moult, and while that is interesting, the question remains: does it breed true, is it genetically distinct, and should it be shown?