Most beginners tend to default to titanium white whenever they need to lighten a color. But when you add white to a color, it not only lightens that color, but it also lowers the saturation (makes it weaker) and makes it cooler in terms of color temperature. That can sometimes work against your color mixing, …
Browns and grays are usually what you end up with on your palette after a long painting session.
To darken a color, the default for most people is black. But you could also use: