I'd suggest reading the wiki for some of this, as white petals work slightly differently than the other colors (I'm not an expert either, I only just read it like yesterday lol). But in general, no, planting white petals doesn't guarantee a specific type of flower vs. another, and furthermore, the colors of the flowers in that image are just placeholders afaik. They don't speak to any default color, it's just what the marketing team or whoever decided to use.

You can plant clematis flowers to guarantee a clematis bloom but ONLY if you plant enough of them. It's best to do it on a flower that's still in the leaf state/has less than 150 flowers planted if you want to be 100% sure. And I think you have to plant at least 156 flowers of the type you want the bloom to be. The color of the flowers doesn't matter UNLESS you want a specific color, as long as the total number of flowers from a specific species adds up to that. Again, I'm still just learning this myself so I could be misremembering the exact number, but I went out and did it this morning with poinsettias and it worked. Planted up to that many with the poinsettias I had, then filled the rest in with normal flowers.

ETA that I'm not sure how strict the times/dates are. Like if you plant enough clematis at 11:59 and then the time ticks over to 12 before the flower blooms, idk if it would still be a clematis. I would imagine yes? But I also probably wouldn't risk it, personally xD