To wiggle color ( randomly to any possible color), use the following expression, applied to a Color property of an effect. For example, you can apply this expression to the Color property of the Fill effect (Render>Fill), which fills all the visible pixels of a layer with a solid color:
seedRandom(1,true); howOften = 2; r=wiggle(howOften,1); g=wiggle(howOften,1); b=wiggle(howOften,1); a=1; [r,g,b,a]
To change how often the wiggles occur, change the 2 in the second line of the expression to a higher or lower number to specify how many wiggles per second you'd like.
To wiggle Alpha as well as color, change the line that reads a=1 to a=wiggle(howOften,1);