It's not an exact science. If you're making something that needs to be strong (eg something very complex, thin, structural, or has to support itself upright etc) then you would mix more gumpaste with your fondant. You can use just straight fondant if it's something fairly simple that will be flat/low to the cake. Fondant will take longer to dry, so if you want something to dry faster you could use gumpaste.
Personally I don't use gumpaste as such. I get gum tragacanth powder and mix about 1tsp of that into about 500g fondant for modelling things like flowers, animals, people. If I want to make something more structured that needs to be very strong, such as a chair, building, or tree, then I make mexican paste and mix that at about 3 or 4 parts mexican paste to 1 part fondant. (Mexican paste dries very hard, but brittle - the addition of a little fondant makes it much less brittle, meaning it can generally take a knock without breaking.)
? recipe for mexican paste here: http://www.patchworkcutters.co.uk/POM/pom_mexican1.asp
If I need a dark colour, I usually add a teaspoon or two of gel colouring in place of some of the water. That way you're not making it too soft by adding lots of colour after it's made.
You are so much better than I am!! 
Here is my recipe:
http://www.globalsugarart.com/product.php?id=23300&name=Mexican%20Paste%20Mix%20by%20GSA
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