Hi,
I have just started to play with fondant as a cake decoration. What I am having trouble with is getting solid colours. No matter what paste colouring I use, when I add it by toothpick to the fondant (store bought), and then kneed the fondant to spread the colour it always turns out marbled and also gets very sticky to the point where some of the colour comes off in my hands. I have tried cooling it in the fridge first before kneeding and using a bit of icing sugar but neither work. Please help! Is it the store bought fondant that is the trouble?

You've got to keep kneading until the marbelling is gone, adding small amounts of powdered sugar until you get the right texture. Adding gel colors adds moisture to the fondant, you've got to even that out. It takes practice to get the texture just right.

I find that sometimes you have to use so much colouring to get the darker shade it just makes the fondant too soft. For deep colours, try buying the already-tinted fondant. Properly stored it'll keep for a long time.
Marbling effect comes from not enough kneading.

you can also use powered colors, they work well in fondant, but if I had a lot to do in the color, I would buy it ready colored. No matter what colors you use, gel, or powdered, you have to knead, knead, knead and knead, and then maybe knead some more.