After few seconds, post liftoff(anywhere between 60s and 90s), the exhaust plume of PSLV turns into a bluish colour. This is quite evident in night launches than day launches(obviously because the brilliance of sky overwhelms the exhaust plume intensity).
Drop in atomospheric pressure must play a role but what exactly causes this cause to happen?
Why does this happen? Is it common for all solid motors?
Picture for reference from PSLV C28 launch: