Weather, climate and soil can affect them somewhat but the words crimson and red when used in describing Clematis usually mean purple.

Rebecca is the most red of the large flowered hybrids.

Niobe, Kardynal Wyszinski, Westerplatte, Ruutel, Rouge Cardinal are all reddish purple.

To answer your question specifically, soil pH does not affect the flower color on clematis.....it doesn't have much bearing on any flower color, even hydrangeas, except indirectly. It is the presence of aluminum in the soil that determines the coloring of bigleaf hydrangeas -- more aluminum produces bluer flower, less aluminum produces pinker ones. It is just that aluminum tends to be bound or unavailable in insufficiently acidic soils, so decreasing pH bumps up aluminum availability.

As BorS notes, "red" or "crimson" with regards to clematis coloring should be taken with a large grain of salt.....most are various shades of purple, leaning more heavily to the wine tones. Certainly not the true red of something like Crocosmia 'Lucifer' :-) The soil can have an impact, as with regards to the nutrients present that may mildly influence color, same with fertilizers. And don't discount light conditions either, as too much shade or too much intense sunlight will affect coloring greatly.

FWIW, I test grew every "red" clematis I could lay my hands on, including all those listed above (haven't tried 'Rebecca'......I gave up before this one came on the market) and none came close to what I'd describe as red or even crimson. All were various purply/maroon/wine colored shades - all lovely in their own right, but NOT red. The closest was C. texensis 'Gravetye Beauty', which I'd dscribe as cherry red.....a lot of pink to it :-)

Here is a link that might be useful: Brewster Rogerson on the 'Kardynal'

Thanks for the great replies. I really enjoyed Brewster
Rogerson's article. Glad to know I had the spelling right!
This is year #2 for the Kardynal and it is lovely, a wine
color is how I would describe it and now I can rest knowing
it will never be "red".