Consider a box containing four balls: one red, one green, one blue, and one tricolor (=red,green and blue) Consider a box containing four balls: one red, one green, one blue, and one tricolor (=red,green and blue). 
You draw one ball from the box. 
Consider the three events:
R = {the drawn ball contains red} 
G = {the drawn ball contains green} 
Y = {the drawn ball contains red and green} 
Are R and G independent?  At first I thought yes, but doesn't getting a tricolor ball affect the probability of R and G? 
Are G and Y independent? I think no because a tricolor ball can be in both G and Y. 

Using conditional probability formulas:
$$P(R|G)=\frac{P(R\cap G)}{P(G)}=\frac{1/4}{2/4}=\frac12=P(R) \Rightarrow Independent;$$
$$P(R|Y)=\frac{P(R\cap Y)}{P(Y)}=\frac{1/4}{1/4}=1\ne \frac12=P(R) \Rightarrow Dependent.$$
