The colour of the machines is not entirely certain.
I consider that they were most likely painted LVA blue.
However on the few photos I've seen, it seems the colour is somewhat darker though.
Some state that they were overall LVA khaki, a colour such as applied on the Fokker D.VII and Fokker C.I was applied. This colour is considerably darker than that of e.g. the Fokker S.IV.

SchemeColour nameFS-numberWEMHumbrolXtraColorVallejo Model ColorVallejo Model Air
Standard #1Overall LVA Blue   AC D05 115 ? X-214?    

Check www.paint4models.com for an extensive conversion table with lots of colour and paint systems.

Fokker SII-SMALL
Fokker S.II LVA .
Fokker SIIA-SMALL
Fokker S.IIa LVA. Former 94,
Shown above left is the early version. At the right the later, final version.
Fokker SIIA-2-SMALL
Fokker S.IIa Ambulance LVA during the mobilisation (from September 1939 till May 1940).
Location and dimension of triangles on the wings is unknown