depends
if you have an airbrush or not
I use washes of watered down acrylic paint like burnt umber or raw sienna. When I say watered down I MEAN watered down. It it always easier to add than subtract when dealing with paint.
There is nothing wrong with your color being bright on the tank, it is just the first step in painting, you are looking to make your tank look "Faded" or bleached from the sun and dulled by the wear and tear, not to mention dust and grime that builds up. Bright pristine paint jobs are step one in the process, just like the tank in real life, you want to roll it out of the factory and give it some aging
you have many techniques you can use to achieve this, honestly I use all of them sparingly, a good MIG filter to start off, them into the dusting and washes.
But the best way to tone down and unify colors is to use washes or filters from MIG productions
http://www.migproductions.com/
you can buy them from just about all online model shops, I am kinda ticked I cannot find any MIG products locally except his FAQ book. I did finds his powdered pigments at a store and bought them all, they never restocked.
But if you would like to try the filter there is a guy with very cheap prices. His website is "Models by Mickster"
http://www.modelsbymickster.com/
or click here to go to the MIG section
http://www.modelsbymickster.com/cgi-bin ... pl?inv=MIG
scroll down to see all the nifty stuff but you are looking for washes, or filters. Filters work really well on toning down and unifying the base color before moving on in your painting process. They are actually created for this specific use, but you would have to mail order them unless you have them locally.
if you don't want to wait for the mail or you have an air brush. I would suggest putting a dusty layer of watered down model paint, preferably acrylic, over the hull. Just a quick pass from a foot or so out low pressure using a light dusty color like mid stone.
powdered weathering pigments can be bought from MIG, but there are also some powdered pigments used to dust, rust or otherwise a model that you can find at hobbytown made by small companies. I can find them here in Dallas Hobbytown with the glues and resins section, but they are out there.
Modelers used to make good use of pastels, like the chalk, they would brush the dust on and that would sufficiently dull down and weather the colors. You can buy a cheap box of pastels at any store that has art stuff, just make sure they are regular chalky pastels and not oil pastels which are more like crayons
But using washes will always be the easiest and quickest way to do it.
There is a link on the MIG site that explains the difference between washes and filters, but I cannot find it now. Good thing I cut and pasted into a word document, I will post that here someday.
but for now here is MIG himself with a small post about filters and how they work
http://www.missing-lynx.com/rare_world/rw02.htm
and here a guy on Amorama describes his experimentation with mixing washes using different substance with pics, nice
http://www.armorama.com/modules.php?op= ... &artid=782