My own Technique has evolved, through my latest Khorne Dreadnought and here are the pointers I have for those who want to try this. Through this I will assume you know how to properly dilude your paints, you are looking for the consistency of 2% milk when doing the color transitions, that's about 60% water to 40% paint for the average GW paint.
Gold color receipe:
Ochre yellow (VMC) Intermediate bewteen Leprous and light yellow
Light Yellow (VMC) intermediate between yellow and white (50/50)
Hmm, well, a whole lot of grey mixes. Basically, I used 5 shades of grey, the brightest ones being Fortress grey and codex grey.
NMM, Flat surfaces
Big Thanks to CRasterImage for making the following picture!
Consider a square, flat surface.
1- I basecoat the model with my median color in the receipe. For gold, I used Leprous brown, steel would be a median grey. The trick is to get your bearings on where light and dark will be. I first determine where the light is coming from, Let us assume top left corner. Let's also assume that the surface is fairly open to the incoming light, not hidden under the model.
2- I proceed to apply the darkest color in the range in the top left corner, closest to the light source. I covered perhaps 25% of the entire square surface, diagonally starting from the Top left corner. Then, the very opposite corner of my surface is given about the brightest color of my range; my Ochre yellow. Why not yellow? Because I only want to use a little bit of the brightest color, so I'll do it at the end.
3- So now, I have a square with dark flesh in a corner, ochre at the opposite, and the central area (including top right and bottom left corners) are still Leprous brown. This serves as bearings for the color blending to come. That way I know where I need to end the darkest or brightest, and I know just how much room I have for the colors to change from leprous to dark flesh or leprous to ochre. Towards darkness, I apply a strip of vermin brown in between the dark flesh and the middle, diagonally.
4- Note that I paint a diagonal streak and while it is still wet, I go back in a zig-zag pattern to blur the edges of this vermin brown painted line.
5- Then, I mix a little vermin with dark flesh, and I blur a new line in between my pure vermin and pure dark flesh areas. You should get a fair transition. To get a great result you need a very smooth transition in color. Either you're a pro and you can wet blend the colors, or you do like me and you make a lot of intermediate color mixes to blurr the transition.
6, 7 & 8- Once you're done, do the same towards the light, down right corner. You should make sure pure leprous brown is still the color present in the central diagonal axis of your square.
9- Lastly, take your lightest color, light yellow, and mix it a little with ochre. Brush it in the corner, in motions going towards the corner. Finally, a little pure light yellow (always diluted) is applied at the very tip.
10- What's left? The all important bright line on top, where the edge faces your lightsource. Use Ochre, and use the side of your brush's bristles to gently run across the top and left edges of your square, because the top left corner is where the light is coming from.
11- Next, use light yellow your brightest color, do the same thing as you did with the ochre, but start in the middle of each edge and go towards the upper left corner of the square. If your light yellow is diluded enough, the lines made will look like ochre turning to light yellow as it gets cloe to the top left corner. Do another pass of light yellow to make sure the edges very close to that corner are pure light yellow. Voila!
That' the technique basically. On each flat surface you paint, identify where the light is coming form (normally the same for the entire model), mark your dark and bright zones, and blur the color transitions in between. After a little practice you'll get a natural sense of where light and dark need to be and it won't be as tricky as it may seem at first.