Hi!
i typically use Americolor Eggshell yellow (i'm not 100% on the color name..it's been awhile but i think that's it)..once i have that mixed well, i add a drop of Fuschia and then some Red. It takes a long time to achieve..and it's a lot of elbow grease kneading and kneading...but, you can see in my sesame street cake photo how the red comes out..(i used marshmallow fondant as well) i use the same concoction with my icings as well.. HTH
PS: i would take a small amount of your fondant and try it out first...and see if it works for you and if you like it...that way you don't ruin a whole batch of fondant 
Ahh, save yourself a few hours and your shoulders, back, neck, etc....and buy it pre colored. Same with black and deep blue. I like Satin Ice. Red, black, hot pink, orange, etc. When I want pastels I just knead a small amount of the pre colored stuff with white fondant, so much easier.
I agree with tinygoose. I almost always buy my red and black pre-colored. SOOOO much easier and faster.
i also agree with buying bright colors pre-made (satin ice rocks), but when it comes to dying mmf i've always used wilton's red gel dye. it usually takes the entire bottle for a batch, but it makes it really red. another trick i used to use with black and red (like CWR41 mentioned) is to add the dye into the melted marshmallows before mixing it into the powdered sugar. just a thought =D
If you use a mmf recipe that calls for gelatin, use red (cherry)and also add red gel color at the liquid stage and you wont knead all day.
A tip I learned at a cake decorating class, use a bit of red powder instead of gel, mix it with a tiny amount of water (just enough to make a paste), and try that. I did an experiment and compared the powder coloured fondant to the gel coloured fondant, it was significantly more intense.
Simple buy you an airbrush machine. Yeah its pricey but believe me it totally worth it. Your fondant expenses go down because you no longer have to throw away fondant you've colored. Just cover and airbrush the colors you need. Plus no elbow grease and all that kneading. I don't even color fondant to cover cakes anymore. I only color decorations.
How long does that take to dry, and once dry will it smear....when using airbrush?
totally agree buy it. I only buy satin ice in red and black and even though it is a bit costly SOOOO worth it...Trying to make it, gets to be a sticky gummy mess. I do have an airbrush too that I use for the red, I just did it for my budweiser cake. It does take a bit to dry. I have put my in the fridge too to help the color set up quicker. Not sure if it is me, but seems to help. You can also mix a bit of vodka in the coloring, and it helps it dry quicker. I got my airbrush from ebay and was reasonable, and TOTALLY worth every penny!!!!
How long does that take to dry, and once dry will it smear....when using airbrush?
If your cake is room temp it will dry fairly quick. I only have problems when I freeze and carve my cakes. Because I cover them to quickly befor letting them fully thaw they tend to sweat. The color has never run before though. I just save so much money on fondant because I don't have to color a lot then throw away the colored fondant. Everyone of my cakes in my album are airbrushed.
I agree with tinygoose. I almost always buy my red and black pre-colored. SOOOO much easier and faster.
I've learned this recently. I was trying to get black for a Halloween cake and it didn't happen. How's the taste of Satin Ice?
If you add your color to the melted marshmallows before adding the powdered sugar, you won't have to knead it in.
If i make it this is the only way to do it, there isn't any extra work except an extra "stir" or two to get the color in.
But typically i buy satin ice pre-colored.
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