Why do the quarks constantly change colors? In a hadron the quarks constantly change colors.They,then exchange gluons to remain color neutral.For example a red quark becomes blue by emitting a red and anti  blue gluon and then the blue quark absorbs it to become red.
Why do the quarks change color at all?
 A: It is not clear how much you know about elementary particles and interactions.
This is the table of elementary particles in the standard model of physics

And these are the forces with which the elementary particles interact and finally create matter as we see it everyday.
The quarks within the proton and neutron interact mainly with the strong force, because they are so close to each other that the other forces are insignificant to first order.
This is a simplified diagram of what happens in a proton, which is made up of three quarks.


The Tiny White Lie: Wikipedia's sterotypical image of a proton as two up quarks and a down quark bound together.

The reality is more complicated  because of the strong force:


Snapshot of a proton -- and imagine all of the quarks (up,down,and strange -- u,d,s), antiquarks (u,d,s with a bar on top), and gluons (g) zipping around near the speed of light, banging into each other, and appearing and disappearing. (M.Strassler 2010)

The quarks and gluons exchange color charge when interacting in the soup in such a manner as to keep the whole proton color neutral. 
The bottom line is that the quarks change color because they are interacting with each other and a sea of quarks and antiquarks within the  nucleus due to the strong force, and interaction=color-charge exchanges also.
(  The interactions are similar but not the same with molecules in a liquid, they keep exchanging photons interacting with each other, but photons carry no charge, whereas gluons carry color charge ) 
A: In Quantum Physics, the basic rule is: if something is not impossible, must be taken into consideration. Therefore if we are talking about quarks interacting we have to take the possible change in color into consideration. 
