Need help Converting an Images Colors to the closest one in a palette

[Aminivibrio]

So I am developing a video game which uses a built-in color palette similar to old MS-DOS games. Because of this, I need a program that will convert all the colors in an image to the closest color in a given color palette. Unfortunately, my game's color palette has more than 256 colors (around 2000), so I can't just convert all the images I want to turn into sprites to indexed mode. The best method I've found so far is to use the Select by Color Tool and manually convert every color in the image to match my palette. Does anyone know a way to convert images to a color palette instantly besides indexed mode?