The black in the tail is actually allowed but not sure if it makes it easier or harder. The under color is certainly linked to the black color. The slate color is linked to the color in the males. Haven't had problems with under color knock on wood.
I think my near term goal is to even out the salmon over the back. Plus working on cushion and overall type.
My problem is the opposite. I culled a lot of birds for under color. My birds tended to have very smutty under color. The birds with too much smut tended to show black in sections they should not.
The smuttiest of them all were also too dark in their body color.
Unfortunately the birds with too much late in their under color also have the best colored legs. Catalanas have slate colored legs. The ones with no smut tended to not have enough black in their tail sections and lighter colored legs.
So . . . the ones I especially leaned towards keeping were the females with no smut, good colored legs, and better tail markings. After considering type first.
Out of around 60 females (I really do not remember exactly how much), I have 8 pullets and starting to lean towards only keeping four of them. To give you an idea how far there is to go with this breed.
I have appreciated this discussion because you have had me thinking about and looking at a breed I know little about. Both of the breeds we are discussing are wheaton based. I have only had my Catalanas for a year, so I am just beginning to get an idea how to look at them.
I will probably keep a reserve of color (under color) , and maybe a hen with some speckling , in case I start losing black in the females tails. Just as a precaution, because I do not know what I am doing.
I think black is a balancing act with these birds. I saw that with the NH. I learned to look at their under color and wing sections. The German strain is so nicely marked (black), that my attention was more on how correctly the tail section was marked. I imagine that in time that they could go down hill if no attention is paid to it.