How to make armor blocks properly merge?

Those separating bars usually appear between adjacent armor blocks when are not the exact same color and/or material. Is it possible that those blocks have a slightly different shade of grey than the others?

To repaint blocks already placed in a different color:

1. select any block from your toolbar so your UI is in "place block mode"
2. Select a color, either by using the color picker UI (P) or by aiming at an existing block and picking its color with Shift+P
3. Aim at the block you want to paint
4. Press middle mouse-button. You can also press Shift+Middle Mouse Button to paint a 4x4 area or Ctrl+Middle Mouse Button to paint the whole grid in a uniform color.