This is how I paint it on rifle stocks in 120mm.
1 start with a sand or brown colour, Anywhere between desert yellow and dark earth, depending on what you what the finished product to look like. The lighter you start the more the 'grain will stand out.
2 Using brown oil paint (a mixture of Raw umber and Burnt Sienna is good) apply this pretty thinly, brushing in the direction you want the woof grain to go.
3 Leave this for a while to start to dry, how long depends on the type of paint. Once it has thickened up a little, use an older, stiff, flat brush and gently run this over the oil paint in the direction of the grain. The idea is to remove some of the paint, leaving lighter areas and simulating the wood.
It takes a little practice, you should experiment to see how thick to apply the oils, how long to leave them to dry etc. At least with oils, if you stuff it up you have lots of time to clean them off and try again.
In larger scales the slight texture of oils is handy, but you'll want to get it pretty smooth in 1/35. You will need to leave this to dry for about 1 week if you use oils