Sorry, Myouse, but, if you're talking about the flowers, there's no such thing as a yellow hydrangea.
There's a greenish flowered version Hydrangea paniculata Limelight but it fades to pink over the season.
Hydrangea arborescens Annabelle is also green but fades to white.

If you want one with yellow leaves, just starve it of nutrients - you'll kill it in the process though. ?

You may have seen a yellow one but I'm afraid they are dyed white ones. There seems to be a fashion for dying flowers - those bright un-naturally electric blue flowers you sometimes see in supermarkets are dyed as well. I dodn't know how its can be done on a bush.

If you want yellow hydrangeas in a vase you can add yellow food colouring to the vase water and they will take it up but you'd probably have to use a lot to get a deep colour..