Unlock Cells Based On Interior Color

[UnreadSix]

Hi There, I have an excel sheet that currently locks all cells except for the ones that allow user entered/selected data (drop down list). I have it set up that if previous cell is blank interior color of that cell is black, if previous cell is not blank then interior color is white. What I am trying to do with no success is have it lock all black cells and unlock white cells as they become available. the current setup has all cells but two initially black and if a user clicks on the black cell it highlights I want to only allow the user to select one of the two white cells and nowhere else. Thanks in advance!