Hi Wizzo,
Looking at the black I see that you used the Humbrol Matt Varnish - it has a nasty habit on dark colours of doing that. I supect that the white 'talc' (to make a matt finish) is involved.
However all is not lost! Get some black & dark brown (burnt umber is the proper nake) acrylic paint. Also some windscreen washer fluid.
Also ensure you have a flat brush to put the paint on with or perhaps two of diferent widths. 'The Works'shops sell some white nylon brushes which will be quite OK, don't use sables the cost too too much for this sort of work.
First put some of the dark brown in a flat saucer shaped receptacle, and then darken it with some (NOT MUCH!) black. Thin it down with water to the consistency of ink, and add a small brush's worth of windscreen washer fluid, which will act like detergent when washing up - it stops surface tension, which allows the thin paint to lie as an even film. Nut detergent - that makes bubbles - not what you want!
Slowly build up a colour - you can when you see the results add some different colours for 'road dirt' (add a sand colour - again not a lot you want it thin), or rust colour - add red oxide/indian red/cinammon brown/burnt sienna to name but a few colour names. Vary them with black or brown: the latter being much better as black colbbers colours too much really and is a dead tone as well.
One final thing - go to a Morrisons Supermarket if you can and get some Johnson's Klear - it is intended for kitchen floors, BUT is in reality a good and cheap satin varnish - that will priotect your weathering as acrylic paint is a bit soft; to kill the remaining shine if you want add a final 'whisk over' of dark brown/black acrylic as above - just one coat.
Now leave for a long while to give it the best chance of a hard (as possible) set.
Weathering is NOT quick, it takes a time to (in full size) get there and equally the smaller scale version takes a time with lots of thin washes of colour that are put on.
Above all enjoy it and go slowly the colour when originally put on is darker than when its dry, so an apparently too dark colour MAY be just right, so experiemt slowly. You can also add acrylic inks (Games Workshop, or an art shop - there is a very nice rust colour avaialable), as well for even more variations.
