V1668 Cyg, V1974 Cyg, LV Vul, FH Ser, and PU Vul type tracks. The difference among the groups may be related to the envelope mass, which is closely related to the nova evolution speed, but was not fully clarified. They categorized 40 novae into one of these six subgroups, depending on their similarity of the tracks in the $(B-V)_0$-$M_V$ diagram. They are not fully successful, however, to explain why each nova has a similar $M_V$ in the $(B-V)_0$-$M_V$ diagram.

Nova light curves show a similar property in the $(t/f_{\rm s})$-$(M_V-2.5 \log f_{\rm s})$ plane. The color-magnitude evolution should also be treated in the same way, that is, based on the time-stretching method. In the present work, we propose the $(B-V)_0$-$(M_V-2.5\log f_{\rm s})$ color magnitude diagram. We call this “time-stretched color-magnitude diagram” in order to distinguish it from the usual $(B-V)_0$-$M_V$ diagram. The time-stretching method is useful because many nova light curves follow a universal decline law. Once their absolute $V$ magnitude is time-stretched, nova light curves overlap each other in the $(t/f_{\rm s})$-$(M_V-2.5\log f_{\rm s})$ light curve [@hac10k]. Here, we stretch/squeeze the timescale of a target nova as $t'=t/f_{\rm s}$ and overlap its light curve to the comparison (template) nova. 