Color constructor
Create a color with an integer representing the rgb value of red, green,
and blue. The value 0xffffff is the color white #ffffff (CSS style).
The rgb
value of 0xffd700 would map to #ffd700 or the constant
Color.gold, where ff is red intensity, d7 is green intensity, and 00 is
blue intensity.
Implementation
Color(int rgb, [num? alpha]) : _argb = Color._rgbToArgbString(rgb, alpha);