After a pleasant but not very photogenic walk along the Seine in Chatou and Croissy on a gray afternoon, I came home via the metro station Réaumur-Sébastopol, which still has one of the Hector Guimard entrances. The only spots of color in an otherwise dull scene were the red lamps over the entrances and a splash of yellow on the metro sign in the background. So I got rid of the other remaining traces of color in my photo! (Technically, I split the photo into two layers, deleted all but the red from the top layer, reduced the saturation to about 10% in the lower layer, and then merged them. So there is a little bit of color elsewhere.)
This marks my first month (or a bit more) of trying for a photo a day. A nasty cold led me to miss a day (and do a pretty lame photo the following day). It has been an interesting challenge.