It does sounds like your son is seeing energys, such as auras or energy orbs.
Let me try and explain what these colors could mean;
Brilliant yellow color: spiritually inspired or experiencing a spiritual awakening.
"spiritually inspired could mean that his teacher loves to teach his/her students in general"
Dark yellow aura with tinges of brown: you're a student of life.
You feel pressured to achieve your goals and score high on tests or in life.
"dark yellow the most common color because of the pressure society puts on everyone"