I might be able to help you at least with direction, but you will need to share more details about what you are trying to do and where it goes wrong.
i open a program called artview, open a morph15front.art file, choose a letter in this file that needs to be replaced, push the load bmp button, choose the letter i need (in bmp file format), push the button to replace, and the program replaces the letter, but the colors change, so the game doesn't show this letter in the game.
Got it. Unfortunately I am not familiar with artview program. The only idea I have is to ensure that your new .bmp file has bit depth = 8. This is important since the game can only read such bmps. You can check bit depth of your bmp file if you go into its properties (right click .bmp file -> Properties -> Details).