Color
See the ColorDemo class.
// Draw solid colors
Gui.Heading("Color");
Gui.Color(color);
Gui.Color(color32);
// Draw gradient
Gui.Heading("Gradient");
Gui.HorizontalGradient(horizontalGradientLeftColor, horizontalGradientRightColor);
Gui.Gradient(Color32.Blue, Color32.Red, Color32.Green, Color32.Yellow);
// Draw a color picker
Gui.Heading("ColorPicker");
Gui.ColorPicker(ref hsvColor);