What is the difference between colloid thrusters and ion electro spray thrusters
They appear to be two names for the same thing. See http://lmts.epfl.ch/MEMS-ion-source, for example.
According to the Wikipedia article Colloid thruster they are the same thing:
A colloid thruster (or "electrospray thruster") is a type of low thrust electric propulsion rocket engine that uses electrostatic acceleration of charged liquid droplets for propulsion. In a colloid thruster, charged liquid droplets are produced by an electrospray process and then accelerated by a static electric field. The liquid used for this application tends to be a low-volatility ionic liquid.
The Wikipedia article includes a link to some MIT lecture notes for Aeronautics and Astronautics; On the Lecture notes page for 16-522-space-propulsion-spring-2015 one can find MIT16_522S15_Lecture22-23.pdf, the title of which is:
Session : Cone-jet Electrosprays, or Colloid Thrusters
These two terms indeed seem to be use interchangeably.