Hi Juan Pablo,
if You are searching for information regarding painting a P40 model, I strongly suggest You to read this topic:
In the same forum there are some very informative topics about building it.
The base colour was the green, not the sand.
Nicola del Bono found one colour photo (and me later another one) that shows that probably (at least in our opinion!) some P40 tanks were painted in 4 colours, not 3.
It seems possible that the tank was painted in a medium green, then received patches of brown and dark green, then thin lines of sand bordering (more or less accurately) the patches.
Of course this may be a wrong analysis of the colour photos, and oll P40 were built in 3 colours, or (as the difference between the patches is not evident in B/N) maybe no P40 was ever built with 3 colours, but all in 4: judge Yourself!
About the exact colours of paints used for Italian vehicle after the italian surrender of the 8th of September, there is not known any written documentation, so I suggest You to refer to the few colour WWI photos of these vehicles and the Semovente M 42 da 75/18 numbered 114, built under German control in early 1944 and rediscovered and unearthed in 2000, and restored by the Museo dell'Aviazione at Rimini, where it is now exhibited since 2004 (if You want to built a 3 colours vehicle).
Up to now no researcher/historian ever stated if the colours used were Italian or German paints.
My personal opinion is that the exact hue is not significative for scale factor, weathering, batches of paint used during assembly, etc. etc.
Let me know if You need any additional information, including the other color photo I removed from the post I linked.