Layer blending mode --> Alpha Add. AE's default Alpah belnding uses a multiplication, so it doesn't add up linearly.
You need of course do so in the pre-comp that actually is used to produce the tile pattern, not on the solid...
I don't think I understand. I put my pattern in a pre-comp?
The pattern is the solid. Do you want me to make each square a seperat layer and set them all to alpha add?
I can't tell enough from your screenshots because you've scaled them down so much and you have not shown us the comp settings.
First, if those really are one pixel wide grid lines you are trying to shot you're going to have sub pixel interpolation unless they are lined up perfectly on the X and Y axis and, if they are 3D layers also the single pixel lines must be exactly perpendicular to the camera and positioned exactly at a distance equal to the zoom value of the camera. Otherwise the pixels will be interpreted, the lines won't be exactly one pixel wide any more, the colors will shift, and the grid will look as it does in the last screenshot.
Speaking of the last screenshot you sent, if you indeed are dealing with single pixel wide lines then the only way to understand what is going on in the image is to look at the comp window at 100% zoom and Full quality. I can't tell if you are doing that from the screen shot.
Rasterizing of illustrator files (vector shapes) has an option in the File Interpretation pop-up. Click on More options and set the anti-aliasing to "more accurate" for eps files. That may help things if all my assumptions are correct and your positioning is exactly accurate.
I also have a question for you. Do you move across this video mosaic? if so, then the single pixel boarders are not going to be prety because of everything that I said above.
If you want to see the diamond grid lines against the white background then you have to make the lines bigger.
If on the other hand, you do not want to see these lines then there is another way to approach your project that involves a square pre-comp that is rotated 45º and used as a track matte to reveal your diamond shaped video layers. The trick here, turn Continuous rasterization off for the pre-comp and the aliased divisions between the boxes go away.