Colored ions indicate the presence of a transition metal. Transition metals display colors because of their incompletely filled d orbitals. As the transition metal absorbs a particular wavelength of light to raise the electron orbital, it causes the light that passes through the solution to emit a color.
Ions usually block one or more small segments of the electromagnetic spectrum. This causes the normally white light to display a color. For example, if light is passed through copper sulfate solution, it appears light blue. This is because copper sulfate solution is a transition metal ion with an incomplete d orbital. Copper sulfate solution absorbs red-colored light from the spectrum to power its electron movement. When it blocks the red light from passing through the solution, the result is blue light coming out the other side.
It can take practice to learn what color an ion should display. Accordingly, chemists have devised charts that explain what color a given transition metal displays. For example, chromium produces green light, while manganese ions produce pink light, which soon changes to brown light. Lead oxide ions produce blue-green light in solution. Titanium tetrachloride ions produce white light in solution. Potassium ions, by contrast, which are not transitional metal ions, produce no color.