SKUSA Rulebook: 22.214.171.124. Panel Colors: Effective 01/01/18, Numbers shall be black on a yellow background, at least 15 cm (5.9”) tall and have a 2 cm (.787”) wide stroke represented with an Arial font. No shaded or outlined numbers are allowed. The competition number shall be bordered by a yellow background of 1 cm (.393”) minimum. They must be in place prior to any official session, on both front and rear panels, as well as on both sides towards the rear of the bodywork. The driver is responsible for ensuring that the required numbers are clearly visible at all times to timekeepers and officials.