Ombre effect on flat, one-piece fondant

you could dust it into ombre-ness

Or paint it...

i should explain that huh

so brush the whole cake with whatever color

leaving a portion of that first dusting untouched

then  for the second go 'round--at the certain point you choose

dust it again all the way down to make a bit darker layer

repeat and on & on maybe even adding in a darker color at the end

to get the darkest color to blend well overal you can mix your dust colors in the first place of course so there's a little bit of everything in there then you get darker & darker with the layering and you can use the solid dark color at the end and it all blends

if that's understandable

It's almost easier to start with the dark and dilute as you go up.  Noting worse than starting and having it be too dark to really hombre it. 

If you paint it, start with very diluted color and pain the entire section that will be colored.  Let it dry a bit and go around again leaving a band of lighter color at the top and repeat until you have the desired effect.  It looks nice painted... watercolor-esque.  Use a wider paint brush like you would use to gesso a canvas and something thinner as you get to the darker area.  A 1 - 1.5" brush would be good. 

If you dust it make sure to use a nice big poofy brush so you don't have a lot of brush lines or darker areas.  Think a big powdered foundation brush.  Start at the bottom and lighten as you go up.

Yes, I would paint it on too, with a wide brush like lilmissbakesalot says. However, I must be one of the few people in the world who likes the ombre effect dark at the top and lighter at the bottom .
 

Research the "Skinner Method." This mixing method was developed for plastic clay, but works well for fondant too.

See example of true ombre on a cake here: http://j-a-m-n.tumblr.com/post/33174657078/ombre-fondant-tres-leches-cake-with-swiss

Sorry for the bad photo.