One theory is that your plant which is forming seed is genetically reverting to their (ancestral) parents’ color: white.

Another is that after pollination, the flower tissues due to its wilting, might be forming some ethylene gas (just like when fruits are ripening) and thus bleach out the color.

This has happened (and particularly) to a few of my blue colored vandas in the past that turned white just before they began to wilt so this might be a natural tendency for some blue colored vanda types. Personally, I suspect this to be caused by the production of ethylene gas. I suppose the blue flowered coerulea are particularly sensitive to this.

Ethylene serves as a hormone in plants. It acts at trace levels throughout the life of the plant by stimulating or regulating the ripening of fruit, the opening of flowers, and the abscission (or shedding) of leaves.