Create 20-Color.java

        // TODO: Change this color to its equivalent gray value.
        // The red, green, and blue variables should all be
        // set to the gray level, which is computed as a weighted
        // average of the original red, blue, and green levels.
        // The weights (corresponding to the sensitivity of the
        // cone cells in the human eye) are:
        // red - 0.2126
        // green - 0.7152
        // blue - 0.0722