Bob,
Currently there is not a button that will simply let you "release" a color that has already been assigned to a block.We have noted this and it is on the wishlist to add this type of feature. However, you do have a few different options to get the result you are looking for.

1-From the Colorization Dialog, pick on our our white symbols so that you get a white fill instead of a color fill.
2-Before turning on the color (or turn off the color before performing these steps)- Freeze the plant layers that you do not want colored, run the colorization tool, and turn the frozen layers back on.

For option 2, you could get as specific as you want, for what plants get turned off. Meaning, you could use the highlight plant tool, click a specific tree/shrub, press "S" to select those blocks, put that selection on a unique layer, and freeze that layer. It doesn't have to be the entire tree/shrub layer.

Please let us know if you have further questions and we can help guide you in the right direction.