The Erigeron you mention is hardy - it forms mats of foliage and flowers, but it needs cutting down in autumn and loses its leaves anyway till the following spring.

If you want something that looks good year round, you will need to consider something like a small evegreen shrub with perhaps some variegation to the leaves, such as Pieris Little Heath.Otherwise, Heather should be fine, but most require acid soil, so that means you'll need ericaceous compost, which you can buy at the garden centre. There's a wide range of heathers available, but choose one with nicely coloured foliage - that's what you'll be looking at most of the year, for although they flower, they don't flower all year.

The other option is to plant it twice a year - once in autumn (now) with small evergreens, (garden centres usually stock a range of small evergreen shrubs which are cuttings taken this year of much larger shrubs just for winter display) ivy and maybe some pansies to look nice over winter, then replant end of May next year with summer flowering stuff, like fuchsia, trailing petunias, lobelia, busy lizzies, there's a wide range of flowering plants sold at that time of year.

In regard to moss, that needs damp soil and shade to thrive, so its not a natural bedfellow for most plants in pots.

Thank you Bamboo! I like the idea of Heather the most. Isn't Heather evergreen as well? Do I need to change soil every year? How about feeding? Do you think it is fine to put some mosses with heather in a same urn?

Yes heather's evergreen - bear in mind that pots need watering. No, they won't need feeding at this time of year in new potting soil, and you can put moss in there is you like, but I wouldn't recommend it. Choose one or two heathers with different colour foliage that flower at different times, if you want heathers. Garden centres often sell packs of six small ones in varying shades, in stock now. Whether you can use six or not depends on the size of your planter. Hopefully your urn has a drainage hole in the bottom - if it hasn't, it needs one or two.

Sounds perfect for me! Thank you for explaining me in easy way!

There are basically two sorts of heather and they will have two words on the label, ones with the first word Erica and ones with the first word Calluna. Ericas flower in winter or spring, and Callunas flower late summer. Callunas tend to get rather straggly if they are not trimmed after flowering.
They both need to be in a sunny position.Both are evergreen but Callunas in particular can look rather boring and unattractive when not in flower. Callunas particularly do best in the sort of compost that is called ericaceous.

Thanks for great information Steragram! I think I like the look of Callunas than Ericas!

They are lovely at this time of year, say August to September, but very boring and rather scruffy the rest of the time.