You should only need to select a new color from the Color Chooser panel in the lower right corner of the screen. Depending on what option was selected for it, it could look like any of the following four options:

If you've already tried selecting a new color from that panel, but the results of the tool you were using still showed up red, it could be related to a couple things. The first thing that comes to mind, is the Sticker Spray tool. Since it's recommended that users create their stickers with a pure red base, any presets that don't have the variations panel set up to use the Hue/Tracing Hue Dial correctly, will likely result in those presets only being allowed to draw red strokes. If you were using a different tool (say, the Oil Brush, for example), but it only allowed you to paint out red strokes, it's possible you may be picking up that hue from a tracing image. Did you happen to load a tracing image, with red in it?
Nothing is easy to the unwilling.