Thank you @lando.schumpich. I missed the bitmap class methods. SetPixel() made the function instant. Python code is bellow.
def Swatch(color=(250,200,100)):
bitmap = drawing.Bitmap(10, 10, drawing.PixelFormat.Format32bppRgb)
col = drawing.Color.FromArgb(color[0], color[1], color[2])
width = 10; height = 10
for x in range(1, width - 1):
for y in range(1, height - 1):
bitmap.SetPixel(x, y, col)
return bitmap