from msdn:
Drawing a Shaded Rectangle
To draw a shaded rectangle from [0,0] to [100,32], define a TRIVERTEX array with two elements and a single GRADIENT_RECT structure. The following example shows a horizontal GradientFill rectangle call.
TRIVERTEX vert[2] ;
GRADIENT_RECT gRect;
vert [0] .x = 0;
vert [0] .y = 0;
vert [0] .Red = 0x0000;
vert [0] .Green = 0x0000;
vert [0] .Blue = 0x0000;
vert [0] .Alpha = 0x0000;
vert [1] .x = 100;
vert [1] .y = 32;
vert [1] .Red = 0x0000;
vert [1] .Green = 0x0000;
vert [1] .Blue = 0xff00;
vert [1] .Alpha = 0x0000;
gRect.UpperLeft = 0;
gRect.LowerRight = 1;
GradientFill(hdc,vert,2,&gRect,1,GRADIENT_FILL_RECT_H);
