Didn't know you're using that program. I have never tried using Paint3D but in case you cannot drag color boxes (black and pink), you can change them by swapping color codes. Take a look at the color codes of those two color boxes. Copy the certain color codes of those colors and paste them in either note pad, word pad, power point, document, or whatever writing/typing document files you have, so you can use those codes to paste into your color table of the program you're using. If you put the color code of pink in the color code of black which is the first color box and change it, the pink background color of that image will become black. So you need to change that other color box (pink) with black which is not the first color box. After that, save it as png like you save it as a palette if you want to use it as a palette for conversion. You really don't have to save/make it as a palette file which I never do.