Frank,
If you mean you want another field to be a different color if your
Yes/No field is Yes, then you apply the conditional formatting to the
control which you want to change. In the Condition you put...
Expression is... [YourYesNoField]=-1
.... and then select the colors as required.
If you mean you want to change the color of the Yes/No checkbox, then
sorry, I dont't think this can be done. The only way I know of to color
a checkbox is to make its border Shadowed, and color the border... but
the border color is not included in the conditional formatting function.