Hi. Welcome.
First of all, you'll need to edit your helmet colors in a 256 colors Bitmap file (BMP file), which has 74 pixels in width x 15 pixels in height.
After you get your helmet ready on the BMP file, you should import the file to the game, using the
GP2 Edit tool. You can download it here:
http://grandprix2.de/Util/gp2edit/gp2ed186.exe, and after that, install it on your machine.
Select your driver on the carset, double click on the helmet image, and than go to "single helmet" and "LOAD". Choose your BMP file helmet, open, and than on the below, click on "Export". It will insert your helmet on the game.
You can use a model of helmet to start the edition of your helmet, if you doesn't have one.
Here is a model of blank helmet, which you can start editing it in the MS Paint:
If your MS Paint is a new version, and if your helmet got on wrong colors when load to carset, try to edit your helmet in the old version of MS Paint: You can download it here:
https://drive.google.com/file/d/1d_S_Mt ... sp=sharing