Some possibilities:

There are variants with lighter flower color, including white, and with the genetic variation inherent in seed-grown plants, yours may remain the current colorful to the particular genes being expressed by your plant.

Some types of plants can show variation in flower color depending on a plant’s age, or the flower’s age, or the temperature, or the amount of sun. I don’t know if Hardenbergia is sensitive to any of these factors.

Where did you get the seeds from, montanabrowne? (Now that I've seen your user name, I figure you might have bought them.)

On bush walks, I see mostly purple-flowered Hardenbergia. White and pink ones are much less common. In fact, when I see one I exclaim, "Oh a pink one!"

OP states the seeds came from their own plant.

Growing plants from seed is always a lottery. they often are never the same as the parent plant. To ensure the same plant, take cuttings