Sometimes the burden of proof needs to be shifted. In this case, heres why. This quote is from the wiki source you gave me.
"When a rock is
contact altered by an igneous intrusion it very frequently becomes more
indurated, and more coarsely crystalline. Many altered rocks of this type were formerly called hornstones, and the term
hornfels is often used by geologists to signify those fine grained, compact, non-foliated products of contact metamorphism. A
shale may become a dark argillaceous hornfels, full of tiny plates of
brownish biotite; a marl or impure limestone may change to a grey, yellow or greenish lime-silicate-hornfels or siliceous
marble, tough and splintery, with abundant
augite,
garnet,
wollastonite and other minerals in which
calcite is an important component. A
diabase or
andesite may become a diabase hornfels or andesite hornfels with development of new hornblende and biotite and a partial recrystallization of the original feldspar.
Chert or
flint may become a finely crystalline quartz rock;
sandstones lose their
clastic structure and are converted into a mosaic of small close-fitting grains of quartz in a metamorphic rock called
quartzite."
So, you see, yes, metamorphic rock can and does change not just form, but color too. A change in form would also logically lead to a change in color. Even if ever so slightly.