Many of the shrubby sages with red fragrant flowers, such as Salvia elegans, are quite hardy and do well in pots. We have several that have survived minus 17 to 20 this winter. They give a colourful display all summer long from the middle of spring.

I'm just trying to visualise what a 6 litre tub looks like, and not managing very well. I'm better with how many inches deep and wide though...
You haven't said whether the area you want the pot in is in sun or shade or half and half - consider Pieris varieties - if its shady, Pieris formosa 'Forest Flame', if not so shady, or in a lot of sun, or in a more exposed position, Pieris japonica varieties such as P. japonica 'Firecrest' or P. japonica 'Mountain Fire'. Both these have bright red new leaves, particularly in spring, and flowers, evergreen year round. Needs ericaceous compost.