Need Serious Help

I tried saving my types chart (sprite with 16colour range) in mspaint as a bmp 16... it made all my colours grey.

Fast forward an hour and I've just edited everything from scratch a second time. I save as a 256 bmp and now I can't get it to open up in NSE.

How do I save my bmp16 as a bmp16 without microsoft paint changing my colours? I've tried googling for the past two hours and nothing comes up.

To be clear, are you talking about the image that has all of the types on it? (What I have attached below.)
Because that has 256 colors in it, and will not look pretty if compressed down to 16.