I do think Colin Powell will face backlash from the Republican party for endorsing Hillary Clinton. While many Republicans have endorsed her over Trump, he will nonetheless be subjected to backlash. However, those Republicans who continue to support Donald Trump regardless of his actions should receive backlash from their party as well.
No, Colin Powell will not face backlash from Republicans for endorsing Hillary Clinton. Even the Republican party, save a few, understand that Trump would be a disaster in the White House. Colin Powell is also a respected member of the military and former member of the Bush 2.0 administration, so the GOP will respect his opinion.
I believe that a significant percentage of Republicans do not approve of Donald Trump as president. Many Republicans have refused to endorse him, and some have even endorsed Clinton. I think there is also a significant portion of Republicans who do not want Trump to be president, but are unwilling to say so publicly. Since so many Republicans agree with Powell, he probably won't face much backlash.
Republicans have been dissociating from thier party and endorsing Clinton for some time now. The RNC is very close to endorsing Clinton themselves. There are very few people who are higher up in the party who are still openly supporting Trump. This says more about Trump than it does anything else.