Ok, first of all, let me tell you beforehand that not all vehicles can be texture modified from it's texture, so don't blame me if you can't find it´s bitmap.
Second, always make a backup of the original texture in case you mess up or want to revert to it.
...and third, shall something go wrong and you have to reinstall, don't blame me neither... do it at your own discretion.
Now that its out the way, let's begin.
Download SAUE (San Andreas Ultimate Editor) from here or here.
Once it's installed, read the readme file to know how... and all it's ready to go, open it.
Click IMG Editor tab at top center, and then click Open Img File on the left.
Select gt3.img file and click open.
The images file will open, and will display all the image files.
Go to find, and type in the vehicle you want to mod. After a few seconds the files will display under it. Select the one you want to mod, eg Hydra.txd
Select the one you want, and notice to the left of the image, will display all the texture files.
Click Edit TXD on the bottom, and it will open another menu.
Leave the alphas alone for now, only focus on the files that are displayed.
Select the one you want and click export.
Then save it to a folder of your choice, preferably one named mods.
Once the file is exported, you can now edit it at your like using a photoshop or paint.
Once you are done, choose Save As, to save in a different file, and retain the original, and give it a different name.
Do NOT overwrite the original.
Return to the UE, and this time import the edited texture.
Import your new texture over the selected texture, and click Exit TXD Editor.
It will prompt you to save. Do it so... and it´s done.
Now try it out.
