wavelength $\lambda_1$ and $\lambda_2$ is defined as [@A'Hearn1984; @Jewitt1986]:

$$S_v(\% /1000 \AA)=\frac{Af\rho_1-Af\rho_2}{Af\rho_1+Af\rho_2}\times\frac{2000}{\lambda_1-\lambda_2}$$

$\lambda_1$ and $\lambda_2$ are the effective wavelengths of the filters: BC\[4450 Å\], GC\[5260 Å\] and RC\[7128 Å\].

We found that the RC-GC, RC-BC and GC-BC colours are redder than the Sun with mean values of (14.8$\pm$3.3), (13.2$\pm$2.6) and (12.4$\pm$7.5)%/1000 $\AA$ respectively (see Figure \[color\_slope\]). This result is in agreement with previous apparitions, with values of S$_v$=15%/1000 $\AA$ in 1985 [@Schleicher1987] and S$_v$=13%/1000 $\AA$ in 1998 [@Lara2003]. These values are consistent with the colour of the nucleus of 21P (12.8$\pm$2.7)%/1000 $\AA$ measured at a heliocentric distance of 3.5 au in 1991 [@Luu1993]. They fall within the range observed for most JFCs [@Lamy2009; @Solontoi2012; @Jewitt2015]. During our long monitoring, we did not detect any significant variation of the colour of the dust in the coma (or any outburst).

Abundances ratios {#abundances}
=================

Studying the molecular abundances and their ratios with respect to the distance to the Sun gives information about the homogeneity of a comet’s nucleus and the chemical processes involved in the coma. Based on the relative abundance of 41 comets, [@AHearn1995] classified comets into two groups based on their C$_2$/CN ratio. Typical comets are defined as those having 