Quickshade is a tinting dip made by Army Painter. The way it works is you block-paint your figure or model, then dip it in the Quickshade and shake off the excess (definitely something you want to do outside or in a large box-enclosure). The shade will run into the cracks and crevasses, darkening the recesses. Once it's completely dry you can dry-brush some highlights and give it a shot of lusterless flat spray.
I don't dip it...too messy. I brush it on, then brush off the excess. It will leave a glossy finish which can be okay for tinplate figures, but for more realistic finish you will definitely want to give it the overspray of flat lacquer to dull it down (and protect it).
Before there was Quickshade some folks used Minwax Polyshade, which you can buy at Lowes or the hardware store in various colors. I've never tried that.
To be honest, Quickshade is not perfect, but it looks pretty good from a few feet away. Google it...you can find it at some mail-order hobby suppliers. I think it comes in three shades.
You can also use inks, such as Game Color made by Acrylicos Vallejo, or Citadel has some tinting inks as well. With these I block paint the figure same as with Quickshade. I take a small amount of water and add a drop or two of the ink (very sparingly). Then I just paint the figure with the diluted ink wash. The diluted ink will similarly run into the cracks and recesses. However, one issue with the ink is that it will darken flat acrylic paint significantly...sometimes that looks okay, other times not. One thing you can do is give the figure a coat of gloss or semi-gloss lacquer before using the ink. This will mitigate some of the ink darkening the lighter colors. Then after it dries give it a spray of the flat lacquer. You have to experiment to find what works for you.
Hope this helps.