The epoxide forms of the carotenoid compounds are the pigments that are so highly valued for their pure, dense and stable yellow color in roses. In all modern roses the genes for these pigments came from R. foetida, along with liabilities of that species.

Alone among all the old garden roses, Fortunes Double Yellow has the ability to produce the carotenoid epoxides. Your success with hybridizing it to Joycie is remarkable given that no other FDY offspring exist and that Joycie is far enough downstream of the foetidas that the nasty stuff is diluted. I hope you keep going with them.

Other partners that you might consider for FDY are Francois Juranville, William Allen Richardson and Reve d'Or. These all accumulate beta-carotene (the starting point for the epoxide forms) so crosses with FDY should occasionally give strong yellows useful for further line breeding of yellows that are not influenced by foetida. They also have high levels of certain other yellow (flavanol) compounds that will sometimes act to give subtle shadings (bronzing, as with Whisky Mac and William Allen Richardson itself).

A more daring approach would be to cross FDY with species roses. Here the idea would be to introgress for non-fading yellows. Brute-force survey crosses with various species would be as likely to give interesting results as any other strategy but I would suggest R. wichuraiana, R. virginiana, R. multiflora and R. pisocarpa as starters. This is because other close hybrids of these species already exist for compatible downstream crosses targeting remontancy, doubleness and the other things necessary to fully tame them.

I agree with the first comment and would like to add a great big WOW!
Extremely appealing to me.