I'm not quite getting the picture of what you are looking for. The juniper IS low growing, prostrate ground cover, 6-8" high. Anything to be a border in front of that would have to be so small or flat as to be nearly invisible.
For summer you could pop some dwarf marigolds in front of it for contrast...
Creeping jenny is a very low perennial ground cover just a couple inches high. It has brilliant yellow foliage that would be a nice contrast and is very cold hardy, but it doesn't really like afternoon sun or drought and it isn't evergreen.
Candytuft (Iberis sempervirens) is a cold hardy evergreen ground cover for full sun, that gets covered in white flowers. But it gets a foot tall, so even though it is considered a low growing ground cover, it would be taller than your juniper and tend to hide/shade it.
The combination of things you are asking for is very difficult.