Sunspots are not different in composition from the rest of the solar photosphere.
They are regions of reduced surface temperature caused by concentrations of magnetic flux that inhibit convection.
Convection is how energy is primarily transported to the photosphere; because the magnetic flux inhibits convection, sunspots get less energy from the sun's interior than other regions in the photosphere, and so can be more than 2,000K degrees cooler-- appearing dark in contrast to normal regions of the sun (although since the spots are still 3,000-4,000K, they still glow brightly in themselves).
Isolated from the surrounding photosphere, a single sunspot would shine brighter than the full moon, with a crimson-orange color
Because sunspots are of widely varying size and are just regions of the photosphere, it's difficult to speak of 'mass' sensibly, but back-of-the-envelope a fairly large spot, with a surface area comparable to the earth, might involve 5 x 10^12 metric tons of gas cooler than the surrounding photosphere.