What is this yellow?
It’s not necessarily an dye that’s available in game if it’s from a mechanic that’s covering you with a color.
For example. Here is the Guild Wars 2 color wheel. The squares are the dyes from back then, but you can see there’s a lot of areas that are not ingame dyes.
You husband can go to the dye panel and set it to show all dyes, not just the one he has, and then set it by hue and go through all the yellows to find the closest match.
ANet may give it to you.
When u get into the arena to Place stuff – all u’r gear turns yellow – its the placer’s color so to say, so that other ppl competing there can tell u apart from them (if they are there at the same time). Their gear will turn red/blue/green depending on team. Cant say what shade of yellow it is though, sry.
Judging from your sylvari’s pants, pastel yellow would be a close match.