Why are the Targaryens White?

Aegon the conqueror's mother was a Velaryon, so he is half Velaryon.
Aegon's married his sister Rhaenys so his son Aenys is also half Velaryon.
Aenys married a Velaryon so his son Jaehaerys is three quarters Velaryon.

Jaehaerys married his sister Alysanne, so his Baelon is also three quarters Velaryon.

Baelon married his sister Alyssa so his children; Viserys and Daemon are also three quarters Velaryon.

How does it make sense that the Targaryens and Velaryons are of clearly distinct ethnicities when they intermarry so frequently?