之前我创建的CompatibleBitmap是使用memDC来创建的(memDC = CreateCompatibleDC)
仔细看了MSDN关于CreateCompatibleBitmap的remark章节才清楚。原来memDC默认识单色的。新的bitmap必须使用hSrcDC来创建，才能出现彩色图。
The following is copied from MSDN
Note: When a memory device context is created, it initially has a 1-by-1 monochrome bitmap selected into it. If this memory device
context is used in CreateCompatibleBitmap, the bitmap that is created is a monochrome bitmap. To create a color bitmap, use the hDC that was used to create the memory device context, as shown in the following code:
HDC memDC = CreateCompatibleDC ( hDC );
HBITMAP memBM = CreateCompatibleBitmap ( hDC, nWidth, nHeight );
SelectObject ( memDC, memBM );