That is because the image is not truly gray-scale. For instance, look at pixel 715,2017, you will see that its RGB value is (33,30,25).
For the next update I have added a tolerance value to IsGrayScale, so this works:
If ImageEnView1.IEBitmap.IsGrayScale() then
ShowMessage( 'Image is gray scale' )
If ImageEnView1.IEBitmap.IsGrayScale(10) then
ShowMessage( 'Image is almost gray scale' )
ShowMessage( 'Image is colored' );