It's normally better to 'build in' the basic fading when painting the model, with specific areas added as secondary coats.
However, on a bare metal finish such as your P-38, this would normally be done by masking, and varying the tones of the metallic paints used when painting, to simulate the different 'grains' and types of alloys visible on the airframe, and any tonal shifts due to the elements or dirt.
Planning is the key to all! However, you could try some gentle dry-brushing on various panels where there are no decals. To minimise paint lifting with masking tape, run a strip of tape through finger and thumb, before use, to remove some of the tackiness. I've used this technique for many, many years without problems, albeit on enamels - don't know how well it might or might not work on less durable acrylics though.
I am masking around the paint and around the larger decals. I put the masking tape on my jeans to lose enough of the adhesive to not cause any damage. The little ones, like Airframes said, I will have to use a brush.
Good stuff, but be careful of removing the tack in that manner - it's possible for the tape to pick up tiny particles or lint, which could possibly damage the paint, especially acrylic. Running it through finger and thumb, whilst exerting light pressure, avoids this risk and, as the edges of the finger and thumb are less prone to grease or sweat, and the tape is moving, the tack is removed without depositing any unwanted particles on the tape.