Ok here goes. Physics away.

When you sneeze your water based saliva forms a sperical blob on the monitor. This is due to the surface tension of water. When the three different colored strips of light hit the saliva the saliva acts as a lens and bends the different light away in a different direction instead of blending them into one beam which you recognize as a color.

The equation for this is:
N1 sin theta1 = N2 sin theta2
Where N is the refractive index of the material and theta is angle of incidence as measured from a line tangent to the surface

r       g         b
 r      g        b
  r     g       b
   r    g      b
    r   g     b
    rsssgsss b  
In the illistartion above you can see that the lens like saliva produces a bending affect away from the perpedicular incidence line because it is moving from a material with a high refractive index (saliva) to one with a low refractive index (air). The reason the individual colors appear swirled or distorted is becuase the saliva's surface if not perfectly spherical. If it was you would observe a regular pattern. And the reason it appears to shimmer when you move your head is because different beams of diffracted light line up differently with your eye as you move your head around.

I would do out the calculations but it would be very hard to make the measurements for this on such a small scale. Anyways that is why you see the three distinct colors.