if you're in a pinch, and already have the bright yellow vinyl handy, you could always laminate the yellow onto a white vinyl before cutting to make it more opaque.
seems odd, however, as I always thought the 50 series was opaque enough to lay yellow onto black to begin-with. I don't have any handy to check, as I use the 3m 7725 & 3630 series vinyls for my stock colors.