  Neptunian Trojans (NTs), trans-Neptunian objects in 1:1 mean-motion resonance
with Neptune, are generally thought to have been captured from the original
trans-Neptunian protoplanetary disk into co-orbital resonance with the ice
giant during its outward migration. It is possible, therefore, that the colour
distribution of NTs is a constraint on the location of any colour transition
zones that may have been present in the disk. In support of this possible test,
we obtained $g$, $r$, and $i$-band observations of 18 NTs, more than doubling
the sample of NTs with known visible colours to 31 objects. Out of the combined
sample, we found $\approx$4 objects with $g$-$i$ colours of $>$1.2 mags placing
them in the very red (VR) category as typically defined. We find, without
taking observational selection effects into account, that the NT $g$-$i$ colour
distribution is statistically distinct from other trans-Neptunian dynamical
classes. The optical colours of Jovian Trojans and NTs are shown to be less
similar than previously claimed with additional VR NTs. The presence of VR
objects among the NTs may suggest that the location of the red to VR colour
transition zone in the protoplanetary disk was interior to 30-35 au.
