Maybe I've missed something... but what if you knit the lettering in white or in a slightly different yellow? Perhaps that, combined with knitting them in purl bumps, would make it stand out enough to be legible.
What a wonderful idea!
Mommy to two wonderful girls
"I don't have to conform to the vagaries of time and space, I'm a loony!" -Campbell Bain (David Tennant) 'Takin' Over the Asylum'
OTN: Cabled Sweater
, arm warmers, kids' clothes