There are 3 distinct JRT coat types, Smooth, Broken and Rough.
A coat that does not have longer hair on the head, face, legs or body. A smooth coat does not require any alterations and really very little grooming. The ideal smooth coat should not be thin(like a whippet coat) or overly thick(this can shed excessively), it should provide protection from the elements. It does shed, especially twice a year in the spring and fall.
A broken coat the is similar to a smooth coat but has longer hair on the head, face, legs or body. A broken coat needs minimal work or grooming, it is a very easy and attractive coat, they will generally only require a quick tidying up. A broken coat should not be soft - the longer hairs are coarse and provide protection from the elements. That sounds simple enough but in my experience with broken coats they can vary tremendously and can change a fair bit throughout the life of the dog. I definitely have seen both light broken and heavy broken but the difference from a rough coat is that you can definitely also see smooth areas of the coat and ears are smooth. The broken coat sheds but I think less so than a smooth coat.
A rough coat has excess long hair on the head, face, legs or body. The hair is longer than on a broken coat. A rough coat requires more grooming than a broken coat and generally is the most maintenance of the three coats. A rough coat should not be too soft or thin as all three coats should protect the dog from the elements. Some my rough coats do not shed at all, others shed very lightly most of the year.
There are a fair few variations in color with the JRT, but the official colors fall into Tan/white, Brown/white and Tri. For show purposes the prominant color should be white, that's to say the JRT is a white dog that has patches of color.
As hopefully you have gathered by now, I do not breed for the show ring. I owned JRT's a long time before they were ever accepted into any kennel club and I continue to breed the type of JRT that I grew up with, in that way MBF Jack's are a unique type of JR.
I will describe the colors that I see in my lines.
Red/White(no black hairs through the color, it is a pure color)
This is registered as Tan/white, it can vary from a rich red to soft gold.
Tan/White(black hairs through the color)
This is registered as Tan/white. The dog often has black around the muzzle, ear tips, base of tail much of which will fade as they mature but some keep the subtle black around the muzzle.
Black/White(no other color)
This is registered as Black/white, it is a simple straight forward color 😊
To produce black and white one of the parents has to be black and white, unlike the tri gene where a dog can be a carrier, black/white will not be produced unless one of the parents is b/w.
Tri, black and white with tan points. The tan points can range from light gold to a rich chestnut.
This is registered as tri, this color seems to confuse people the most.
To be considered tri it must have 3 distinct colors that do not blend or mix together.
There is a pattern to tri color on a Jack Russell, the points are on the eyebrows, cheeks, underside of the ears, sometimes depending on the darker markings placement around the bottom and the underside of the tail.
If a dog is a carrier for the tri gene they can produce it even if neither parent to the pups is tri.
It is a very striking color. If a dog is not a carrier for the gene they will not produce it unless bred to a tri and even then they may not.
Brown/White, this color can vary greatly, often there are some black or dark hairs throughout the coat.
This is registered as Brown/white, I find consistently that pups that are dark brown at birth fade to a lighter brown as they mature.
This coloring tends to be dominant.
So these are the 5 colors we have here.
Some dogs carry 'the fading gene' which means their coloring will lighten quite dramatically, like you see with Iris in the middle. Her coloring was all dark as a pup. Marty and Peaches on either side of her, Mother and daughter do not carry that gene although both lightened somewhat as they matured.
Pugsley carries the fading gene, as his father was a tri and he lightened a lot as he matured.