To be fair, some gearbox oil is red (e.g. Automatic Transmission Fluid which is also spec'd for some manual cars).
It could be the rear seal.. but I've no experience of this issue so I can't add much... but anyway, jack the car up onto stands and then crawl under it. Trace the oil (or coolant) upwards ... either to the radiator, the hoses or thermostat (if its coolant) or to the driveshaft..
Assuming it's oil.. look behind each driveshaft (inner CV) cup, since that's where the driveshaft seal is. If there's oil around the cup(s), then that seal is the culprit.
The rear seal I presume is the input shaft seal on the gearbox... in which case the oil will emerge from the joint between the engine block and the clutch bell-housing (next to the sump) so you can't get confused with which seal is leaking. The chances of gearbox and driveshaft(s) leaking at the same time would be quite unlucky..
Coolant leak is most likely a split radiatore... but the Younger Mrs S' Cinquecento had a habit of loosening its hose clamps for ages... (eventually when the radiatore split I fitted new rubber mounting bushes and that fixed it) so it could be a hose leak. Thermostats can also leak.. usually followed by splitting apart if it's a cheap replacement.. so have a look at that. Don't be tempted to fix a coolant leak with Radweld etc. It's only a bit more expensive to actually fix the leaky and you won't have cooling or heater problems.
Ralf S.
