//This works fine:
//(The result of PdfView1.RenderPage is a Bitmap, created from the displayed PDF-page)

procedure SaveToBitmap;
var
	Bitmap:									TBitmap;
	
begin
	Bitmap := PdfView1.RenderPage(0, 0, PdfView1.Width, PdfView1.Height, ro0, PdfView1.Options);
	//allow only bmp-Filter:
	if SaveImageEnDialog1.Execute then begin
		try
			Bitmap.SaveToFile(SaveImageEnDialog1.FileName);
		finally
			Bitmap.Free;
		end;
	end;
end;


This ends in a B/W-image on saving as PNG- or BMP-File; Jpg is OK:

procedure SaveBitmap(Sender: TObject);
var
	sExt:									String;

begin
	//ievPDF is TImageEnView; LegacyBitmap has no influence
	ievPDF.Bitmap.Assign(PdfView1.RenderPage(0, 0, PdfView1.Width, PdfView1.Height, ro0, PdfView1.Options));
	//here looks still OK:
	ievPDF.Update;
	if SaveImageEnDialog1.Execute then begin
//nothing:
//		sExt := Lowercase(ExtractFileExt(SaveImageEnDialog1.FileName));
//		if (sExt = '.png') then begin
//			ievPDF.IO.Params.PNG_Filter := ioPNG_FILTER_ALL; 	//ioPNG_FILTER_NONE; 	// ioPNG_FILTER_PAETH;
//			ievPDF.IO.Params.PNG_Compression := 5;
//		end;
		try
//nothing			ievPDF.RemoveAlphaChannel;
			ievPDF.IO.SaveToFile(SaveImageEnDialog1.FileName);
		finally
			ievPDF.Blank;
		end;
	end;
end;

Bitmap := PdfView1.RenderPage(0, 0, PdfView1.Width, PdfView1.Height, ro0, PdfView1.Options);
ievPDF.Bitmap.Assign(Bitmap);
//also ievPDF.IEBitmap.Assign(Bitmap);