Again double base 1st

[DownTownTonyBrown]

Same as what Greymule says for FED. The double base is not intended to create new restrictions as to who touches what; it is intended to stop the collisions as the batter-runner runs through the base.

If the play is coming from the foul side then the defense can use the orange and the offense should use the white so as to avoid a collision. Casebook play 8.2.12 helps a little to clarify this.

Additionally, in FED, if the runner doesn't use the orange base when she should (play/throw is coming from the diamond side of the double-base), it is an appeal play. The defense must appeal that the runner essentially missed the base. If the defense is correctly on the white and there is a collision because the runner also used the white (only) then the runner is out for interference.

Here are the FED rules:
The Batter-Runner is out
8-2-12... If using the double base, and there is a play on the batter-runner, the BR, touches only the white portion and the defense appeals prior to the BR returning to 1st base. This is treated the same as missing the base.
Exceptions
1) If the ball is thrown from the foul side of first base line, or
2)when an errant or missed throw pulls the defensive player into foul ground.

8-2-13... If using the double base, and there is a force play by an infielder on the BR, who touches only the white portion and collides with the fielder about to catch a thrown ball while on the white.

Penalty: Interference is ruled, the ball is dead, the BR is out, and all other runners are returned to the base occupied at the time of interference.

I realize that your scenario was initiated by a fielding error rather than a throw but to me the intent of the rule seems... fairly... clear. I think if the defense plainly beat the runner (as in your situation), she should have been called out.

Hope that helps