Painting a me 109 with the usual mottle free hand but getting overspray onto the light blue how do you get rid of this or make it less pronounced dry brush?

Okay, for correction...try getting in super close withthe blue (or whatever base color) and outlilning the spots. Yeah, I know tedious PIA.
As to preventing it, pratiec getting the right combination of pressure and viscosity on scrap and again, get in super close with the aiirbrush top.
Hope this helps.

If the blue is the underside You could try masking it with some tape or simply hold an index card under your airbrush in front of the blue part to prevent the overspray....