If the throw pulls the fielder into foul ground you DO NOT want the fielder crossing the bag to the white portion with a runner approaching causing a collision.
We agree on this point and I already stated that.
What we don't seem to agree on is the definition of
"off the base and into foul ground".
My understanding is that if the fielder is set up on the white base to receive the throw, but the throw is "off" only enough to cause her to shift her feet directly to the colored base, that is not to be interpreted as "off the base and into foul ground".
Suppose that the fielder is set up on the white base to take the throw. Where is the runner going to be running? Straight to the colored base (we hope). If the fielder shifts over just enough to be on the colored base, the fielder is now in a direct collision course with the approaching runner. Yes, runners may switch over to the white base when the defense uses the colored bag, but in the case of an errant throw it is likely that the fielder would be moving over at the last second, too late for the runner to switch over to avoid her.
On the other hand, if the fielder is completely off the base and completely in foul ground, such that she must "come back" to the orange base, the runner should have time to recognize this and be able to avoid the fielder.
Allowing the fielder to use the orange base when she is completely off the base and completely in foul ground allows both players to prevent a collision.
Allowing a fielder to move directly from white to orange puts the fielder right in the runner's path and makes a collision more likely.