With my eye both on the finished building, as it appears on the
Artitec site, and on the list of colours supplied by the same manufacturer, I first tried using something different for the roof than the colour recommendation dictated. Since Artitec used a copper-ish finish, and since I already had 2 small bottles of Tamiya XF-6 (Copper) I first used the formula below:
64 drops of XF-2 (Tamiya Flat White)
20 drops of XF-6 (Tamiya Copper)
87 drops of
X-20A (Tamiya Thinner)
The result above wasn't really what I was after - the glossy effect actually ruined everything. Hence I turned to the suggested Artitec colours, bought the Tamiya XF-56 (Mettalic Grey) which is the corresponding color from Tamiya for the suggested Artitec list (they only give Revell and Humbrol colour codes) and used it, while trying to leave a hint of the previous coats. Below the result, after the roof was secured to the rest of the building.
Formula used the last time:
20 drops of XF-2 (Tamiya Flat White)
20 drops of XF-56 (Tamiya Metallic Gray)
40 drops of
X-20A (Tamiya Thinner)
[later edit] This is the color ZK on the Artitec instructions