Color flow breaks down in the prescence of grease.
Since we use crisco when handling fondant I do not recommend doing it directly on the fondant. It is better to do it on paper and let it dry.
Good question! I have never had any luck with color flow. I am curious to see what others say!
I tried to make color flow trees last night and they didn't harden very fast. How much time do you have to give it? I let them go over night, put them in a tupperwhere to take to my office today and they melted in my car.
I guess it isn't a good idea to use heat with color flow pieces around. 
They shouln't have melted unless there was some grease on them. Mind you I have never tried heating them up. It usually takes around 48 hours for an average piece to dry. If it is a large piece it can take longer.
I will keep that in mind. Now I know I didin't wait long enough to remove them. I have a couple left for another cake that I will make in a week so they will be fine then. Just have to watch how hot my car gets!! 
Also, make sure not to put the color flow pieces in the fridge...or any place cold enough that they can form condensation when brought to room temp. The condensation will make them really weak and cause them to break down...happened to me..learning experience. LOL.
Good point!! Another tip I was taught is to pipe a color flow blob (bubble) near your piece that is similar in depth to your piece using your leftover icing. If you do a few of them you can test the blobs first for dryness to see how close your peice is to being dry. Always let them dry at room temperature and away from direct sunlight as colors will fade in the sun.
The heat in your car should not have caused a problem. I use a very warm lamp to dry my run sugar so it dries shiny, not mat, and that does not cause it to melt. The other poster was right about the grease being the problem.
By the way, color flow is the Wilton brand name for run sugar, or soft sugar. It's basically soft royal icing.
there is no problem doing a run sugar piece directly onto a fondant covered cake or plaque. If im making an extra large piece that im afraid i may break moving, then i will make a fondant or gumpast plaque first and pipe directly onto that. the grease has to be mixed in to it for it to fail.
I have also done a large color flow peice on a cookie, like the shield cake in my photos, thats another option.
I tried to make color flow trees last night and they didn't harden very fast. How much time do you have to give it? I let them go over night, put them in a tupperwhere to take to my office today and they melted in my car.
Did you close up the container you put them in? If so then you cut off the air and they need air to dry. They do need atleast 48 hours (sometimes longer depending on the size and the humidity in your area. You can quick dry them with a heat lamp as was said, or you can use your oven with just the light on.
As far as putting it on fodant, go for it. People use royal icing on fondant all the time. The last two years I have made dummy cakes for the convention. I have covered them in fondant (that I did roll out on crisco) and one of them got borders and pieced attached with royal icing. I still have it and the royal icing is just fine. The other has a color flow piece made with royal that still looks as it did when I made it.
I am so glad to learn that. I was just answering with my limited knowledge from Wilton. I love this site. I learn something new every day.
Thanks Ladies!
Quote by @%username% on %date%
%body%