Lightroom does not appear to offer a built-in way to change the crop grid color. A practical workaround is to create a virtual copy, reduce the exposure enough to make the white crop grid easier to see, set your crop there, then sync or copy only the crop settings back to the original image and delete the virtual copy. It adds a few seconds, but it can make precise placement—such as lining up rule-of-thirds points—much easier on very bright photos.