One die of red colour one of white colour and one of blue colour are placed in a bag.
One die is selected at random and rolled its colour and the number on its uppermost face is noted. Describe the sample space 

A die has six faces that are numbered from $1$ to $6$ with one number on each face.
Let us denote the red, white, and blue dices as $R,W$ and $B$ respectively.
Accordingly when a die is selected and then rolled the sample space is given by,
$S=R_1,R_2,R_3,R_4,R_5,R_6,W_1,W_2,W_3,W_4,W_5,W_6,B_1,B_2,B_3,B_4,B_5,B_6$