No links but I have had good experience with various methods. First off are the Woodland Scenics rail paint felt pens, they work but tend to be a bit too 'orange'; they also can wear out the tips pretty quick. I am currently using very diluted acrylic paints, mixed to get the colour I want and applied using a small paint brush allowing the liquid to soak into the ballast and running the brush along the rail sides. I have also seen an airbrush used to good effect, blending in various shades by varying the number of passes and the size of the spray paint. On a big layout the airbrush saves a lot of time, but probably overkill on a small 009 layout like mine.