In photoshop your grayscale colors will change as soon as you save it to a BMP.
Say you set some beach area (rgb color of 170,170,170) in a layer above another layer with say water (rgb color of 0,0,0). Then you set some farm 1 (rgb color of 68,68,68). When you save the file to a BMP (doesn't matter if you save direct, or first index and or if you flatten the image then save as a bmp the colors will shift.
- 170,170,170 shifts to 151,151,151
- 68,68,68 shifts to 49,49,49
So for photoshop when using multiple layers:
1) Make sure you are working in grayscale mode.
2) Save your work to a PSD file. This is important because you want to save your individual layers for later work.
3) Then flatten your image.
4) Then go and sample your individual an area (beach).
5) Then click on image and select color range from the drop down menu.
6) Make sure the fuzziness is set to 0 and then click okay. This will select all of that color in your image.
7) Change your color value to 170,170,170 in your color palette.
8) Click on Edit in the tool bar and then fill. Make sure the Use: says Foreground Color and then hit OK.
This will replace any pixel that had a color value of 151,151,151 to 170,170,170.
Repeat the process for the other color areas you set. After you do this then save as grndtype.bmp.
4) After doing this click on save as, select the format BMP and then save it as grndtype.bmp.