Dragon Ball GT doesn't have a corresponding manga. The story was developed specifically for the anime series. It's not based on a pre-existing manga like the original Dragon Ball and Dragon Ball Z.
Dragon Ball GT has a manga, but it's not considered as canonical as the main Dragon Ball and Dragon Ball Z manga series. It was created based on the anime rather than the other way around.
It's a bit tricky to say for sure. The number of DBZ manga can vary depending on different editions and compilations. But you can find detailed information on dedicated manga websites or forums.
The 'A GT Project' manga and hentai are most likely not related. Hentai is known for its adult nature, but the 'A GT Project' manga could be anything from adventure to fantasy, and not necessarily related to that kind of adult content.
The colors in the DBZ manga are often chosen to match the mood and atmosphere of the story. Sometimes they're used to highlight key elements or to make certain characters stand out more. It really depends on the specific version you're looking at.