Mixing in the colors is just to get the right shade you are after. Start small, and maybe write down an aproximate amount of color used. Remember, the spackle is white so it takes a little, then a little more, them a little more, then some more, to get things as dark as you may want, then you have to add a small amount of another color or two. If not it looks just too flat and uniform and too brand-spanking-new. Before I knew it, my mix was kinda soupy, maybe like runny ketchup (??) but since September I can report no cracks. There is a base layer, then another 'coat', which I sanded smooth-ish, then a top coat for good measure, and sanded that too. Total thickness is maybe 1/16", maybe less, not sure.
Ok, thing to remember, mix it in the train room, where the lighting will dictate the proper color you are after. My batch upstairs in the kitchen looks a bit too yellow, but perfect for slightly-aged concrete in the train room. Use salsa jars or whatever has a decent lid- you want to mix up enough, but you want more than enough leftever so that you can let the coat dry and sand and generally see how it looks. Drying can affect the final color. A good jar with a lid keeps things good for other coats/touch-ups. I can honestly say that now in December the left-overs from September are still ready to go in my salsa jars.
Also, take you have and put some on a scrap slab of your road base- plywood, styrene, foam board, whatever, so that you have the exact same colors to experiment with for weathering techniques. Handy also so you can move it around the layout and see how light affects it!!
First I started with black. Black+white=gray..well, yes and no. It was asphalt, not the concrete color I wanted, so back to mixing other colors. I know I have a yellow in there, and a dark gray, a buff color for sure, and probably even a hint of Barn Red if I rememeber correctly. I'm pretty sure I got a dark Hunter Green-type color in there too, in an extrememely small amount. Taking just one tiny dip of a popsile stick's tip in the paint and get most of it off on the sides of your tray, then swirl it in. Do that for the red and green for sure. Subtle change, but believe me, you notice it if it isin't there! Aged asphalt has that charcter too, at least in my area.
To top top it off I didn't make quite enough, even though I thought I had, so I mixed more with my same formula. It was slightly different naturally, so that second (non-black) batch became the base coat, even though I had wanted to do it all as one coat. I was sure to sand LIGHTLY what I had down to smooth things out a bit, then added my third (final) batch over it, and sanded that too. Done! Took my right-angle (hobby square) and after marking off the appropriate scale distance between them, took a mechanical pencil and just lightly scribed in my lines. No need for a wash or anything to bring those out, that mechanical pencil (.7mm I think) has the consistant size and IMO color. I did have a few low spots that I didn't want to sand out. Probably would have made the road to unrealistcally lumpy. These areas were right near the tracks, as they are in real life, so the original too-dark batch (rememeber railroaders, rule #2...never throw anything away) which I had put in an empty salsa jar became an instant and real-life patch. Sanded that smooth, and there ya go. Haven't done my lines yet, but got ahold of a white paint marker, wide-tip. No one around here had anything but the fine tip for weeks, so I had to wait until they finally re-stocked them. I got one and ran test and it does indeed work perfect, as others on the internet have stated. You see the concrete color show through perfectly, so it is naturally weathered, so to speak. I doubted my airbrush ability to run dark tire lines for anything other than a foot, and my airbrush has wanted to splatter lately, through no fault of my own I'm sure, so I experimented with weathering powders. The colors I came up with working the best are Ash, Dust Bowl Brown and Grimy Gray. Used small cheap makeup sponges on a stick, from my local grocery store of all places. What I have now is a main drag. There will be a "side street" if I ever finallize the building placement, and I'm sure Iwon't have enough spackle/paint to cover it, but that isn't a problem. I still have the third batch in a jar so I can match things close enough, and besides, since this will be a side street the color would be off a little anyways. Maintanence schduals vary from street to street.
