To prevent moire the angle of the lines on the halftone image need to be at the right angle to the screen mesh. With four colour printing, the angle is different for each colour. There will be one colour which gives a moire pattern and this is generally the yellow printer. The moire is not noticeable because the yellow is a lot weaker than the other colours. You can visualize this by rotating your halftone positive on the screen so the pattern disappears and use that angle when exposing your screen emulsion.