There is no known way to change a clematis' color that I am aware of. There have been some people that postulated that you might get different shades of blue by altering soil pH but I have not seen any documented studies showing this to be true. It is often difficult to remember the color of a clematis flower from one year to the next.

I have noticed subtle color changes on clematis blooms over a growing season as the season turns from cooler temps to hotter temps. I know that this year a lot of the PNW has had extreme cold and a small bit of extreme heat thrown in for extra measure. That might have affected your blooms more so than any nutritional or additive effect from the soil.

When they come from a greenhouse, the blooms are often not the same color as they are when grown in a garden. My Niobe is definitely purple. Reddish purple but not a true burgundy or red. As a former florist who did a ton of weddings, I'm very picky about color names.