Can you clarify what colour difference you're seeing? It could simply be discolouration from circulation, especially on the top one where it's quite worn. Keep in mind that scanners don't always make accurate colours.
It's possible they were exposed to the sun or light for a length of time. Yellow is always the first colour to fade. Can be pretty sure it's not a printing error unless the colour was missing completely.
Don't argue with an idiot, he will beat you with experience.