I think some of the best garden displays come from a mass of the same plant grouped together. I don’t often practice what I preach being a well known doter-in who squeezes plants into a small space.
These black viola ‘ Molly Sanderson’ look almost black when clumped together and the light is bright. The plants self seed and last for a few years.
Your massed plants do not need to be in the ground, a group of pots can get a vibrant display together. It is an opportunity to experiment with colour matches and variety.
Perhaps given the ‘massed bands’ title I should have shown some trumpet flowers!