I wasn't sure if he meant yellow cards
Or the band Yellowcard
I can only assume (and you know what that can sometimes mean) that he actually meant yellow cards.
Yellow has been a part of Topps color palette since they began with 1951 Topps Red Backs card game pieces.
Topps would continue to use yellow as a strong background color choice in multiple sets in the 50s and 60s.
This 1958 style is a 2019 Topps Archives card. I have a few different border parallels of this card, but this particilar copy is the base, which I received for free at the September 2019 Philly Card Show where I met McCutchen.
Many more sets over the years would feature yellow, but my first experience with yellow in the card design was a 1986 Rick Rhoden.
The first set I collected was 1987 Topps, but my first packs I had were 1986 Topps. The Pirates had a yellow name printed atop the card.
The most BOLD yellow card I have is this 2018 Topps Heritage High Number
Speaking of backs
I have shown off the 2017 Topps Heritage Bright Yellow back parallel in the past.
These are very rare Short Printed cards, but not nearly as rare as the SSP Gray Back from 2017 Heritage
Topps has produced baseball cards for nearly seven decades.
Like many other printing processes from those days they have used printing presses that utilize printing plates to create the images. There are four colors used to create each of the baseball cards we love and enjoy. The colors are black, cyan, magenta, and yellow.
Here are 8 of my yellow plates. I currently have 26 yellow printing plates of Andrew McCutchen.