i vote yellow.
it's the biggest contrast in colours.
the gray matches the red chip's edge spots and its white edge spot matches the white chip.
the pink is too much in the same level as the red chip imo.
I think the grey/silver look best with the cash chips especially if $25s and $100s are on the table, then pink, then yellow. The color is about as subdued as the rest of the cash set; the pink and yellow are really eye grabbing. It's also related to both the $1 and $5 chips, with the white edgespots matching the base of the $1, and grey matching the edgespots on the $5s.