Although we often think nowadays of Gothic cathedrals having always been constructed of bare stone, many Gothic churches instead had painted interiors, with the paint used to emphasize architectural details. One of the cathedrals which we know was painted was Chartres.
Over the years, most of the paint has worn off or been scraped away, but enough survived in dark corners to give an idea of the original color scheme. The majority of the church was painted an ochre color, with white lines highlighting architectural details like ribbing and pillars. Additionally, white lines traced out the outlines of stone blocks (although they did not necessarily correspond to the actual blocks underneath, and in fact grew larger in size the closer to the ground they were). More details can be found in pages 54-55 of the second link below.
A project begun in 2009 has now restored the painted plaster by removing newer gray
paints and layers of filth obscuring the plaster and retouching it (around 80% of the plaster survived, according to the article I found). The result can be seen in the pictures in this post, all from the article.
However, although it is undeniably good to know what the interior looked like when it was first created, I am not entirely sure I agree with this restoration. If its purpose is merely to change the look of the interior to be more aesthetically pleasing, then there is an equally strong claim for leaving the cathedral unpainted. On the other hand, if its purpose is to be more historically accurate or give a better impression of what medieval churches were like, then it will be only one step in a restoration that can never go all the way, in part because the medieval mindset can never be fully recovered (for more about this, read pages 55 and following in the second link). Additionally, many cathedrals (and not just Gothic ones) went through many stages of construction spanning decades or centuries, and are even now sometimes in a state of change. To hold Chartres, a building which is still very much in use and therefore beholden to the ideals of today’s Catholics of what a church should be, to the exact appearance it had when first built seems too strict to me.