planning to put this on kitchen island

Anything, really! You don't have to match the vase. I assume you want to use fresh flowers, and you can always pick up a new bouquet every few days at the grocery store or local market. Have fun with it!

i want put some artificial flowers.

Am long stem blue or purple flower would look striking against the yellow.

dark blue or light blue

fake blue hydrangeas?

just blue hydrangeas alone or combined with any other flower colors