It's not likely the lens. All lenses diffract light of different wavelengths at different angles, but that should either already be handled by the optics in the machine or it's small enough to ignore, or both.
Here's what my manual says on the matter: "Make the X, Y and theta adjustments for the red and gree colours. Keep an error within 0.5 dot at the center and within 0.8 dot in the circumference."
That is to say, every single technician who sees that the red, green and blue dots that make up the one white dot in the center all overlap by half the size of the pixel in the center of the screen, and by at least one fifth of a pixel at the edge will say the unit is in perfect convergence, or at least up to factory specification. "Perfect" alignment is really impractical.
I'm sorry to say white next to black will always have a colored edge. It's even true of laptop LCD panels because red green and blue never overlap at all, but then they never move relative to each other either. It happens on plasmas and direct view CRT's too. The colors just don't overlap.
Maybe it's just time to accept the imperfection and move on. Don't look at a display of single pixels anymore.
Adding to tahustvedt's comment, 1/800" will move a panel an ENTIRE pixel. You want to move it, what, one tenth of that?
Be at peace, brother.
Chris