Yes, the STL files is simply a set of triangles, nothing more. Colour is done in the slicer, and you're in for another learning curve there.
In Creality Print, it's call painting, and they is a paint icon at the top. This is a bit limited, but gives some options, like layer level painting, or fills etc. Plenty on gayTube about Creality Print painting, which will explain better than a paragraph or 2 here. I find it limited, and a pain to edit if you need to make a change, and often end up clearing all painting and starting again. But its handy for first steps.
Additionally, and more usefully in my opinion, you can also paint at an object level. Objects can be seperate models (though thats a dumb idea, as will cause massive pooping), or text that you add via the Text button at top, or "modifiers" (right like the model, then select modifiers, and the modifier shape). The latter allows SVG file imports, so great for logos and simple pictures.
When painting at the object level, you need to change the view from "Global" to "Object", which is an option on the left of the screen, and it will list the colour against each object or modifier, which obviously you can change.