Wasn't a filter. You copy the image, use the threshold adjustment and choose a setting with large areas of colour, colour this a lighter colour than the other colours you plan to use. Copy the original image again and do the same thing, choosing a threshold with less colouration, then change the black areas again to a colour slightly darker than the other, cut this out and paste it as a new layer without a background fill and place it in line with the lighter layer before it. Keep doing it working up the darkness of your colour for each layer. The one i chose had a white background, three shades of grey and a black top layer.