Sorry, boxing the paint is a technique for painting large areas whose name comes from using a large box in which to store and mix your paint.
In industrial environments, like the US Navy where I learned the technique, you often have to paint large areas, like, say the sides and superstructure of an ammunition ship. And you want it all haze grey. Despite quality control mechanisms, every can of haze grey paint isn't the same colour.
You can't visually tell the differences up close. But when you apply paint from different cans, or God forbid, different batch runs and look at the ship from a long way off, where you used different paint, you see the contrasts. This effect is called "zebra striping". It is not black and white contrast, but it is obviously visually noticeable.
(You can also see differences in colour when you touch up an area with different paint of the same colour or when you touch it up at a later time after the original paint has aged. To avoid this, you use a technique called "walling" where you repaint an entire area up to a joint or bend in the "wall" (called a bulkhead. This technique is used with intentionally slightly different colours in interior design to emphasize corners and wall to ceiling transition. It's like doing an accent wall, except with a subtle change of colour. This is also useful for wargame buildings to emphasize wall transitions. Paint parallel surfaces with one coulour, then add a teeny bit of light grey or dark grey for the perpendicular ones. But, back to boxing …)
So, to avoid zebra striping when you paint your ship, you "box" your paint. You get a large metal box that will hold ten or so five gallon cans of pain. You put eight cans in in and mix them up. Now you have a composite colour that is the average of all eight cans of slightly different haze grey.
Start at one end of the ship and paint one direction. Don't use multiple teams spread out to get it done faster. Once you use about three cans of paint, the box is about half full. Dump another three cans in and mix. Then keep going. You now have a slightly different colour haze grey, but it is 5/8 match to the old colour. Since these colours are only very subtly different, your eye will not see the transition. Since you are painting progressively, you actually slightly recover the last bit of still wet paint and you mix a transition on the surface. The result is what looks like a constant colour, but really is a subtly transitioning array of colours, with no harsh line borders between them.
But in my case, I was not trying to get the same colour, but different colours. But what I did want was the smooth transition. I didn't want a band of blue green on the bottom, a band of green green in the middle and a band of yellow green on the top.
I started my box (a large container cap) with metallic green mixed with a bit of metallic blue and started painting from underneath. Once I had used a bit up I added some more metallic green and kept going up progressively. Once I got a little over halfway up, I started adding metallic gold to the paint instead. This migrated the colours on the top toward yellow green.
Hope that helps.
-----
I copied this post to a threat on the painting board TMP link so if you want to discuss the painting technique rather than the Cthulu project, that board might be a better place.
