The temp cars are placeholders for players entering a game while you're on the track so your PC doesn't temporarily freeze while loading the graphics & 15,000 polygons for a vehicle. As NitrousOxideUK said, you can replace the greybox temp car with something else that's more visible. My advice for a replacement is to use a low-poly car so you don't have a hiccup when the temp car loads. You could also reskin the temp car in pink or orange to make it more visible.
If you have the nametags turned on (Tab key), then the greybox temp car is easier to spot from a distance.
To have the real car replace a temp car, escape back to the pits/monitor and then go back to practice/qualifying/warm-up. No need to leave the online session.