Here is the receipe, this is only cheapo Nescafe

(not soy sauce

), the cheapest from any super market shelf.
I used about a teaspoon + a mug of boiling water, exactly the way how one would make a hot morning coffee (sugar and cream optional but I did not as I was not sure if the parts like sugar and cream.... and as the 91 car was meant to be lean and mean, I think they better stay on diet

)
Then simply dip the parts in while it is hot. Leave them there may be all in all 3 minutes, I did not even care to look at the clock, just check about every 30 seconds. Dip them in tap water to check color or stop the process.
I dyed the parts in a small disposable lunch box. Small parts first then the arms and legs after. Reheated the coffee in a microwave when I found it get a little cold, no need to be pipping hot.
Then as a final silly touch, I put all the parts in the lunch box for may be a little more coloring. As you can see in the photo below, some parts are on top some are fully submerged, this way a slight variation in color is obtained and I think it is more 'natural'.
You can see in the last two photos, the original rc10 bulkhead was lost among the stained parts and the Shapeway arms stood out clearly from the dyed parts
Can't be easier and cheaper than this. Best of all, can drink the coffee after the job is done
