Is it appropriate to ask colleagues opinions when reviewing a paper? Several times my supervisor has asked me for my opinion on papers he is reviewing. I find this helpful as it encourages me to read things I otherwise would not, helps me understand how to review articles, and can lead to interesting discussions. Presumably my supervisor also finds it useful to have a second opinion or sometimes to act as a sanity check.

Recently I was wondering is this entirely appropriate. Not least because all the manuscripts for review have confidential for review only in bold at the top.

So my question is what are the legal/ethical issues with asking a student/colleague/supervisor's (depending on your position) opinion on a paper you are reviewing?

Note I'm not saying that they write the review just that you get them to read the manuscript and ask their opinion on certain points.
I think that this practice violates the usual "confidentiality agreement" that presents (formally, or customary) in a review process. However, I would agree that this is a common practice, and for some people it is just hard to work on their own, although technically you are supposed to give your own opinion on a paper you review.

I would say that if you really need to show the paper you review to someone else, you should check the following boxes:

make sure the person understands that they are looking on a paper under review, and agrees to maintain the confidentiality, i.e.: not to talk about the ideas from the paper with someone else, not to produce own work based on these results until they are made public, etc.
 The actual material paper, and the file, do not change hands: do not send the pdf manuscript to your colleagues, do not leave the printed paper with them for a while.
 Ask specific question(s) about the paper, for which you need second advice, not just a general opinion. Remember, that you should review the manuscript yourself. Definitely do not use the second opinion to shake some work off your shoulders and go.