Inevitably a (very) light shade of blue, to avoid confusion.
The "new" colors in a 4-color deck are blue and green, so they both should be as light-coloured as possible to avoid confusion (especially under low lighting when peeping) with black and dark red.
Aqua.
To me, the gold looks too washed out and low contrast against white.
But I also feel the red/hearts and green/clubs could be lighter/more vibrant to pop. Could be just my monitor though.
(fwiw, I've never been a fan of the darker "security red")
And considering that hole cards are viewed in shadow...
maybe something like this version on the bottom row. The green still needs more vibrancy though
I don’t particularly like that aqua color but it has more contrast than the gold. I would actually prefer the gold but it’s too close to the skin tone on the art work and washes it out. Maybe if you could add more contrasting colors in the artwork itself and leave the pips gold?