Hybrid flowers of all types are crossed with other varieties to obtain particular colors. It is possible and highly likely in my opinion that your clematis's heritage has a white variety in the lineage and its just showing its ancestry. Another possibility is the soil conditions. Although I have never heard of a clematis doing this but some plants flowers are sensitive to pH of the soil like hydrangeas. Pink or blue hydrangeas will flip colors depending on the soil pH. Acidic soils make blue Hydrangeas turn pink and alkaline soils make pink hydrangeas turn blue. My bet though is its an ancestry issue. May I ask what variety of clematis you have?
