I am doing a semi truck cake with fondant (my 1st ever) & need to know how to thin the paste coloring to write with it.

Add it to your buttercream and thin with corn syrup

HTH

You can thin paste color with vodka.

Colored royal icing works well for writing on fondant.

I have used the buttercream to write on fondant.

You can thin paste coloring to gel coloring consistancy or use gel colors right out of the jar with a paint brush to write onto fondant.

Or you can do what I do.. use gel colors and thin them some with vanilla or some type of alchol. They will dry fast and the thinner makes them not be globby