If you're doing your workout correctly, you should be totally burned out at the end. That said, your body needs rest and recovery to get over a cold, and getting exhausted from a hard workout doesn't do anything to help that.....but it CAN definitely hurt it. I recall running a hard, fast 10k with a bad cold back in the Spring of '82. The bad cold progressed to Summer-long bronchitis within a few days of that run. So I'd say yes, you can still do SOMETHING, but back off and make your workouts lighter. Don't push yourself as hard as you normally do. Now that I'm an old goat, if I feel something coming on I lay off my workout completely until I feel better. Thankfully, that's a rare occurrence - I very rarely get sick because when you get to be my age, you've built up antibodies to every microbe known to man!