the colored plates come from olympic weightlifting.

    0.5kg / 5kg  white
    1kg / 10kg   green
    1.5kg / 15kg yellow
    2kg / 20kg   blue
    2.5kg / 25kg red

makes it easier to count the weight on the bar quickly (the metal collars are also 2.5kg each). the green 10kg plate is the first full size plate (ignoring youth/beginner plates).