a friend asked me to make this cake plus cupcakes but I don't have the airbursh. So how can I make the dirty stain on the cake? maybe brushing with some dark color dust?

Also, how can I make the flames with double colors? 

thank you!

two tone flames -- try a striped bag like this:

http://theicedqueen.blogspot.com/2012/03/piping-roses-with-striped-edge.html --

and i like to pipe my stripe/s down into the empty bag as well as the fill color-- you have more control that way --i do a favorite leaf in the autumn with 4 stripes filled with gold icing--one of my favorite things to pipe -- a changing leaf --

not to mention you can buy piping bags that are all set up to stripe --

you can get black petal dust and brush that on--you can also get black cocoa from king arthur flour among other places and that would be cool too

♥

AJust dry brush some cocoa powder on for the grime, or edible black powder. Make colored buttercream and mix the two just barely so you see the color variation when you pipe it.

thank you both! I'll post a pic of the result :)

I used cocoa powder on one of my cakes to give it a "dusty/dirty" look.  I used a fluffy brush, dipped it in the cocoa and tapped off the excess, it works great.  I used this technique on my baseball cake that I  posted if you want to see how it looks.  Good luck.