LacZ staining on tissue extracts

[gtk10583]

I am interested in staining for LacZ in tissues from a transgenic mouse
overexpressing a b-gal transgene.

I have tried mincing the tissue in a digest buffer containing triton x-100
and clearing the extract via centrifugation.  However, the assay I am using
for b-gal expression is a colorimetric assay using ONPG as substrate.  The
problem is most tissue extracts are heavily pigmented and mask the color
generated by enzymatic activity.  Is there a better protocol for extracting
protein from whole tissues for lacZ staining?