Best in this case is leaving a gap. Coloring overspills quite a lot into other neurons, so you have to decide:
Letting in, could it create a merger? (in this case no, overspill does not reach edge of cube, so AI will not extend) and how is smoothness of the branch (quite jagged in this case).
Removing: how big is gap (small in this case). Bigger part of the segment belongs to other neuron.
A scythe decided to leave the gap (current consensus).
If you have questions you can always ask in chat, normally a scythe will be around helping you faster than going over the forum.