svg_color_constant is simply an enumerated list. The colors are defined here. The list contains all of your expected colors, such as black and red. The list contains one extra color element, blank, used when you need to pass a color, but would not like it to show up. This comes in handy for defining defaults for functions, for example.

using namespace boost::svg;

svg_2d_plot my_plot;

svg_color_constant my_const = red;

my_plot.background_border_color(my_const);
my_plot.background_color(lightgray);

	Note
	svg_color has a constructor for svg_color_constant, so you can use a svg_color_constant in place of a svg_color and it will be implicitly converted. However, there is not currently an svg_color::operator=(svg_color_constant) overload, so svg_color my_color = red; does not work.

You can define a svg_color using two different constructors

// The parameters are red, green, and blue respectively.
svg_color(int, int, int);

// Use a pre-existing color constant.
svg_color(svg_color_constant);

	Important
	Any integer value is accepted by the SVG standard, but negative values are rounded to 0, and positive values > 255 are rounded down to 255.

using namespace boost::svg;

svg_color my_white(255, 255, 255);
svg_color const_white(white);

BOOST_ASSERT(my_white == const_white);

	Note
	svg_color's constructor takes in three integer values. The SVG 1.1 standard allows any integer to represent an RGB value, with values less than 0 and greater than 255 being rounded to their respective min and max.

Constants are defined in an enum, svg_color_constant, in alphabetical order. This facilitates quick lookup of their RGB values from an array. Anywhere that a svg_color can be used, a svg_color_constant can be used, as the conversion is implicit.

All color information is stored in RGB format in a svg_color struct. The rationale for storing information in RGB format is because it is precise, and is always representable the exact same way. Storing a floating point percentage introduces the possibility of rounding error, which I would like to avoid at all costs.