According to Covergirl, you won't need a topcoat with these shades - here's what they had to say about the range:
"Dare to go topless with our new nail polish formula that combines a base coat, high gloss color, and a chip-resistant top coat in one simple step."

Covergirl Outlast Stay Brilliant – 300 vio-last is a gorgeous violet crème, which applies like a dream. You could easily get away with one coat, but I usually use two for good measure. It is also so glossy that it doesn’t really need a top coat; of course, that isn’t going to stop me from using one anyway! I can’t help it - I just love the way my nails look with that little bit of extra gloss. It does also tend to give your manicure that little bit of extra staying power.