Not sure if you then want to process the data raw (BMP format is very easy to read) or use libraries like https://metacpan.org/pod/Imager

Using Imager getpixel

if raw and you want to read the pixels yourself, read: The simple explanation first: http://paulbourke.net/dataformats/bmp/

Now the more complex explanation: https://en.wikipedia.org/wiki/BMP_file_format

I could not find the exact post, but I know it exists on PM, it was for BMP with pack. This should do as a start: BMP file manipulation

Just know that with AutoIt3 you can do all this with 1 command: https://www.autoitscript.com/autoit3/docs/functions/PixelGetColor.htm