really important is to use a base color with spray - this avoids the ink from being removed with your nails for example later when playing with the painted units.
Dissolve more the ink on water to have a smoother surface after painting.
only use 3 colors to tanks.
with planes and ships you only need to use 1 color and one shadow, using a second color only for details (ex:the front of zero is black and the rest of the plane is dark green or white)
camouflage in tanks was made depending the theater: jungle many diferent greens together, desert just a kind of yellow
camouflage on ships was made using one single color (for example a kind of blue/green for usa ships
camouflage on planes was made in 2 dimensions: below the plane a color of the sky - light grey more frequently (someone looking to the flying plane from ground would see the sky and the below part of the plane). The other dimension is above the plane - someone on another plane flying over that plane sees the above part the plane and the ground - for example the upper part of the usa dauntless was painted blue so that someone looking down would see the blue from the plane and the blue water of pacific ocean.