For quick color I usually throw in a bunch of blooming - or about to bloom annuals. You could also get a perennial that will bloom in late spring/ summer - after the phlox is done.
Its hard to say where to put it - I think it has alot to do with personal pref - some people like to just randomly plant - while some like a more uniform look.
How much total sun do you get in day? If it's partial sun (8-5 hours), is it morning, mid-day or afternoon? How much water does the area get? If you know the sun and water specifications, it really helps determine what will actually survive. And of course, what zone are you in? Zone 6, 7, other?
Any color of cone flowers would look right at home there. And black eyed Susan would be real pretty too. For a pretty blue you could add Stokes aster. I have these in my butterfly bed. I also have Montauk daisy (nipponanthemum nipponicum) and larkspur for white. The Montauk daisy is a real pretty, neat mounding plant that gets about 2 feet tall. The larkspur gives a nice informal look to the garden. All of these plants are perennials. My butterfly bed is out in full sun and we do get HOT down here. These flowers are all taking the heat in stride. I like your rocks--wish I could put some in my flower beds but the fire ants just love building their mounds around them.
Those elephant ears survive zone 7 facing south, full sun? Wow. I would recommend, since it looks like a dry area, echinacea (purple coneflower) some coreopsis, carnations, maybe daylilies. Shasta daisies, rudbeckia (but not fulgida, use goldsturm)... they can all take it dry and neglected.
Since its a small garden, one nice 'Knockout' or 'Home Run'rose would really look nice. While you are waiting for the creeping phlox to fill in, a few low annuals could finish it. Another idea is salvia 'May Night' and Coreopsis 'Moonbeam'. They look wonderful together and seem to bloom forever if you dead head them.
How about a bed of drought tolerant, practically no-care succulents? Succulents come in a variety of heights and various colors (not just green). Very little care is needed and very little water which is great for beginner gardeners. You'll be surprised at how great these look...
so i went to the garden place yesterday fully intending on buying something to brighten up this garden. after hours of looking and going back and forth between different things, i ended up with dark purple petunias. then at the last second i just put them back, said to heck with it and didnt get anything. i was just trying to picture it in my head and i just have no idea where to put them. it seems like it will just be out of place. i definately need something, i agree with that. argh. this shouldnt be so difficult, should it?
wow. my elephant ears have grown since i posted that. its kinda cool to have pics and see the progress like that. my maiden grass still looks about the same. i was hoping it would be doing better than that.
well, i havent got around to adding any color. i still cant decide on anything. im thinking the reason my maiden grass isnt growing may have to do with the fact that this spot floods very easily. take a look at the pic, it rained last night and this morning. the sun has been out for atleast 4 hours now and there is still standing water. is this a problem and if so, what can i do?
The elephant ears will LOVE the extra moisture. I had some up in my garden where it gets wet and they got to liking it so good, they tried to do a takeover so I had to move them. But a lot of ornamental grasses don't like their feet wet. I'm not sure about maiden grass but I'd almost bet it will resent too much water. Maybe you could move the grass elsewhere and put some caladiums in there--that would give you lots of color.
If you are thinking white I have seen all white flower gardens they are really pretty. Daisies, white sedum (blooms late summer early fall), Anemone nemorosa 'pleniflora' is a very pretty white ground cover that blooms in July. If you want to have a low growing shrub that will be green all year around with either pink or white flowers (from Jan. to april in my zone) Erica is very pretty.