"The shirt is available in white or blue."
Translation:La chemise est disponible en blanc ou en bleu.
My question and mistake too. I am going to guess that these colors do not directly describe the shirts, as a blue shirt, la chemise bleue, but are nominatives, which maybe hide the elided "in [the color] blue or [the color] white. OK, not good, since we have la couleur... Maybe someone who knows will chime in soon.
"En bleu et en blanc" use "bleu" and "blanc" as the masculine name of the colors. They are not adjectives and therefore do not agree with "les chemises".
To have "bleu" and "blanc" agree with "les chemises", you need to use these words as adjectives:
- Des chemises bleues et des chemises blanches sont disponibles = Bleu and white shirts are available.