To have a probability of drawing a marble of any given color equal to 0.2 (or 20%), that means for any color, the ratio of the number of marbles of that color to the total number of marbles must equal 0.2.
Since the total number of marbles is 25, we can calculate the number of marbles of each color needed to achieve this probability:
\[ \text{Number of marbles of each color} = 0.2 \times 25 = 5 \]
So, we need 5 marbles of each color to achieve the desired probability of 0.2.
Thus, the best model for this situation is:
a bag with 5 marbles of each color.