The blue, yellow and red (on fourth down) lines are added by computers that scan for certain colours (i.e. the green of the gridiron), then replace the necessary pixels with the required colour. The final image is then televised. Note that these lines are not visible on the field itself to spectators, coaches, etc., and there are sometimes difficulties with adding the colours (for example, snow or mud discolour the ground so the computer does not recognise it as such, or a uniform is almost the same colour as the ground). In these situations a technician can normally fill the gaps manually.