Yes, you can grow them from seed but because they are hybrids themselves, you will not be certain that the seedlings you get from seed of these plants will come true to what they should look like. Seeds from hybrids may give rise to seedlings that look like the parent plant or they may look entirely different. If the plants from which the seeds are harvested is cross-pollinated by an insect, there will be no telling what the plants will look like but this is how new hybrids are found and introduced into the market.

Only the species clematis will come true from seed and that will be the cause if they are selfpollinated. If they are cross-pollinated by insects, you won't know what the plants or flowers will look like until they bloom.

If you want a specific type of clematis plant, then buying one propagated via vegetative cuttings or layering is the only way to be certain that you will get what you want (provided of course whom you purchase the plant from doesn't mix up labels when they are growing the plants out)!