Rule of thumb is: above the neck symptoms, train (if lightly), below the neck, don't train.

So basically a runny nose won't keep me off the bike. Fever will.

When I have a cold I actually like to go for a ride. Getting my circulation going a bit faster also allows me to... hum, decongest and let go of a lot of mucus. Good time to practice snot rockets. I will feel much better after. I just make sure I wear my most absorbent, softest gloves.... (!)