G'day, Paddy.
That's a brilliant bit of woodwork you've done, mate. I'm watching and learning.
As for the figure, he'd be wearing field grey trousers and cap with coloured piping and band (
http://www.warrelics.eu/forum/imperial-germany-austro-hungary/ww1-german-field-caps-feldmutze-26117/), and an off-white shirt. (Black shirts may have been issued to mechanics, however the black shirts had four buttons- the style on your figure resembles one of the styles of white shirt). You can use Humbrol 111 for the field grey and any off white for the shirt. The boots and belts black or very dark brown leather (where the raven's oil has worn), no idea what colour the braces are. The band and piping on the cap could be either medium blue (train troops- where an old SNCO mechanic like him probably started their career) or, if he'd come over from a combat arm, green, red or even (probably very dirty) white. Not sure what colour the fliegertruppen wore, though, as I don't have many reference on WWI uniforms.
As he wouldn't be wearing his No 1 clothes when working, I'd lighten the Humbrol 111 with varying amounts of Humbrol 28 and/or 147 (28 on the upper thighs, seat and knees, 147 elsewhere) to show fading, and perhaps add a bit of Humbrol 72 (khaki drill) to the knees and seat to show where they were stained from kneeling and sitting on the ground. I'd also lighten the cap more than the trousers and add some darker grey around the top rim, to show staining from the hands.
Cheers.
Dal.
