Color in RGBA Mode: glColor

glColor3f (1.0, 0.0, 0.0);  /* the current RGB color is red: */
                            /* full red, no green, no blue. */
glBegin (GL_POINTS);
    glVertex3fv (point_array);
glEnd ();

voidglColor3{b s i f d ub us ui} (TYPEr, TYPEg, TYPEb);
void glColor4{b s i f d ub us ui} (TYPEr, TYPEg, TYPEb, TYPEa);
void glColor3{b s i f d ub us ui}v (const TYPE*v);
void glColor4{b s i f d ub us ui}v (const TYPE*v);

It sets the current red, green, blue, and alpha values.
If you do not supply an alpha value, it's automatically set to 1.0.

Floating-point data types should typically range between 0.0 and 1.0, the minimum and maximum values that can be stored in the framebuffer.

Unsigned-integer color components, when specified, are linearly mapped to floating-point values such that the largest representable value maps to 1.0 (full intensity), and zero maps to 0.0 (zero intensity).

Signed-integer color components, when specified, are linearly mapped to floating-point values such that the most positive representable value maps to 1.0, and the most negative representable value maps to -1.0.

Converting Color Values to Floating-Point Numbers

Suffix	Data Type	Minimum Value	Min Value Maps to	Maximum Value	Max Value Maps to
b	1-byte integer	-128	-1.0	127	1.0
s	2-byte integer	-32,768	-1.0	32,767	1.0
i	4-byte integer	-2,147,483,648	-1.0	2,147,483,647	1.0
ub	unsigned 1-byte integer	0	0.0	255	1.0
us	unsigned 2-byte integer	0	0.0	65,535	1.0
ui	unsigned 4-byte integer	0	0.0	4,294,967,295	1.0

This value used by glClear to clear the color buffers with glClear( GL_COLOR_BUFFER_BIT ).