This is where all the transition metals are found on the Periodic Table of Elements. The interesting property of transition metals is that they have vibrant colors in solution or compound form. The explanation for this is revealed in Bohr model of the atom.

In Bohr model of the atom electrons can be displaced to a higher shell, but only by absorbing a quantized amount of energy. Since the lower shells are too far apart, the electrons wouldn't be able to absorb the requisite energy from ambient light. But in the d-shell, only a small amount of energy is required, and visible light meets that small requirement.. When the electron absorbs the quantized light, it goes into an excited state, which means that the electrons are not in the stable position or ground state. The electron immediately drops back to ground state and emits whatever amount of light was absorbed. This is where the vibrant color comes from. Since every element has a different electronic configuration, each element will have it's own characteristic color.

While all transition elements (metals which are colored in solution) are found in the d-block, not all d-block elements are transition elements. The reason for this is simple. In the last group of the d-block, all of the d sublevels are already filled, therefore there is no place for electrons to jump. They cannot jump to the f subshell, and since they can only absorb quantized energy, they are unable to jump at all.