Top: radial g-band surface brightness profiles of M 101 taken along different azimuthal selections: azimuthally averaged (solid gray line), along the northeast side (green dotted line), along the east side (orange dashed line), and along the west side (blue dot-dashed line). Shaded regions represent uncertainties. A schematic picture of M 101 has the same color coding as the profiles. Three profiles along the northeast, east, and west sides show large differences at the outskirts. Bottom: radial color profiles. The black solid line is an approximation of g − r color from the Eq. (10) of the text.