If I wanted to write on fondant or even buttercream... would wilton food coloring work? Like do I have to dilute it a bit with water or can I just put the paint brush right into the coloring and 'draw' on my cake???

I have never tried to paint/draw on BC but I have used Wilton to paint flowers on a fondant cake. Don't use water with the coloring on fondant. You need to mix it with either lemon extract or vodka.

I have painted on my fondant cakes with icing colors thinned down with either vodka, or lemon extract. It will take a little bit of time to dry, but they really look cool once done. You can even sponge it on over a stencil just like you would do a wall.