Ok sorry if this gets a little long, but I hope this is helpful.
I think it might be useful to look up color theory, if you aren't familiar with it (basically just how colors are related to each other, names of various color schemes, etc). This might be a good starting point. Something else that might be good is looking at art or designs you like and just using the color picker tool to pick out the main colors and make palettes out of them (I have a few I made myself below). This might seem really basic, but it can be a good way to find out what you like, what works well together, etc. Then you can think, oh I want to make a color scheme kind of like this one, or even base the palette off of another image (when I say image, it's not necessarily just copying someone else's design; you can make color palettes from paintings, photographs, movie screenshots, anything). Also, I think if you aren't sure yet what you want the main color to be, you might want to think about making it a less saturated color (you don't have to of course, but it might be good to think about). Bright colors can be great, but when it comes to placing them next to other colors, it can be a little tricky. It's very easy for them to clash, or look out of place together.
I also included some sample ideas for how to choose a different color, and of where to put the markings, but of course these are just ideas. I think the main thing is to just play with it and see what you like, and above all, to have fun! I hope all this was helpful, n that you can come up with something you like! ^^