Hi Hisham - you worry for nothing

In the Nuts & Bolts book Vol.15 on page 26 there are 2 photo's of a Marder III belonging too Pz.Jg.Ers.Abt.4 in Borna.
It is overall dark yellow with a cross on the side armour. Later on in the book there is another vehicle belonging to Pz.Jg.Abt.37 - 1.Pz.Div in the same overall paint colour.
As for interior colours , I would say you could go both ways.... have interior and exterior dark yellow , or exterior dark yellow and interior shield dark grey!
Regarding camouflage painting - I would suggest that you get out a piece of plastic card and spray paint BOTH sides.. one side dark yellow and the other Pz grey.
Next when its dry experiment with a round number 1-2 brush with very diluted paint. Pick up some paint and wipe it off on some paper or cloth. Next gently stipple the painted card in a small area. Do this several times over the card. With patience and practice you should be able to develop a camouflage technique that will give you good results when combined with weathering.
Best regards
Paul