This function adds an RGBA colour attribute to the new vertex format being created.
IMPORTANT The attribute is stored as four bytes in the vertex buffer and turned into a vec4 (floats).
vertex_format_add_colour();
N/A
vertex_format_begin();
vertex_format_add_colour();
vertex_format_add_position();
my_format = vertex_format_end();
The above code creates a new vertex format with just colour and position values and then stores the format in the variable my_format.