Turmeric is a yellow color due to its most active component curcumin. All of the beneficial effects exerted by turmeric have been found to be because of this component.
Piperine is one of the principle actives found in black pepper. In the body piperine inhibits an enzyme known as CYP3A4. This enzyme usually metabolizes certain drugs and compounds, and in some cases does this so rapidly that it renders many of them somewhat redundant. By inhibiting CYP3A4 piperine can massively increase the oral bioavailability of compounds, so that the body can actually use them.