ひらがな
Your demand might be partially met with DrawPolygon(), the library's innate function.
If you are thinking of a way to draw some basic shapes filled with colors in a gradient fashion, this should be the solution.
Otherwise--to literally "fill" some specific pixel areas--MakeXRGB8SoftImage() might be effective.
In the latter case, I'm afraid you must implement the pixel manipulation algorithm by yourself.
A sample in which a gradiently coloured rectangle is drawn is shown below:
VERTEX v[6];
ZeroMemory(v,sizeof(VERTEX)*6); //Clear rubbish
v[0].x=100.0;
v[0].y=100.0;
v[0].r=128;
v[0].g=255;
v[0].b=160;
v[0].a=255;
v[1].x=200.0;
v[1].y=100.0;
v[1].r=255;
v[1].g=128;
v[1].b=160;
v[1].a=255;
v[2].x=100.0;
v[2].y=200.0;
v[2].r=128;
v[2].g=160;
v[2].b=255;
v[2].a=255;
v[5].x=200.0;
v[5].y=200.0;
v[5].r=64;
v[5].g=255;
v[5].b=32;
v[5].a=255;
v[3]=v[2];
v[4]=v[1];
DrawPolygon(v,2,DX_NONE_GRAPH,false);
I'll be glad if you find this information helpful,
and please forgive me for possible use of rude English.