Metis and Adrastea are the two inner innermost moons of Jupiter. Being deep within the radiation belts, they are difficult to study with a spacecraft. The problem is worsened by the fact that they are only 20 and 12 km in diameter, respectively. Metis was viewed several times during the Galileo mission. Here is a collection of all the angles the spacecraft got on this moonlet.
Using data from other orbits, I was able to colorize the lone image from Galileo's 26th orbit. It appears reddish, likely due to deposits from Io. It was taken from about 290,000 km at 3km/pixel.
Adrastea was the worst observed of the inner moons, the best galileo views simply show it as a tiny disk.
This image below, from NASA's planetary photojournal, shows how Metis and Adrastea fit into Galileo's ring scheme. They are different from Amalthea and Thebe, the sources of the Gossamer ring, in that they are more icy, less heavy in silicates. It is not know whether they are of different origins, or something caused these little moons in their inner orbits to lose their volatiles. It is possible that the rings and all four inner moons are pieces of the same parent body.
Perhaps NASA's upcomming Juno mission will be able to make some improvements. Its focus will be on studing Jupiter's internal structure, but it will orbit very close to Jupiter's cloudtops, likely allowing better study at least Amalthea but maybe, just maybe, some of these even less-known inner moons. Perhaps they are the leftovers of a collision - perhaps they were once as prominent as Saturn's rings. Here is a view of the rings from Galileo, with Europa in the background. This hard-to-explore area calls out for exploration.