The Nova is fading overall, but the Hydrogen emission lines are relatively constant. So as the light output of all the other colours fade, the red of Hydrogen alpha and to a lesser extent the blues of Hydrogen beta and gamma dominate the overall colour, which explains the apparent reddening.
Attached is a low res spectra of the nova from 30th Aug which hopefully demonstrates this, and shows how almost all of the light is coming from the red H alpha band.
Novae Delphini 2013 - Pn a66 58 - Changing luminosity and color ?
