My question is sill not be answered (hard?) and I am editing it to make it more clear.
What I did is:
1. Using BitBlt function copied the whole screen of 256 colors to a picture box picture1.
2. Have checked the picture in picture1 by my eye and found that was perfect.
3. Did savepicture picture1.image, "file.bmp".
4. Open the file.bmp by Paint and found some colors changed to black.
(I have tested that if the picture is loaded from a bmp file, using savepicture can copy the picture to another bmp file without any color changed to black.)
My question is: If the picture of a picture box is copied by BitBlt, How to save it to a bmp file without any color changed to black? (VB3+Win95)