What are you making a portfolio for?
Typically you want your portfolio to be all the same. When we did portfolios, in my senior portfolio photography class, my prof wanted ALL pictures to be color, or ALL to be B&W, etc.
You would therefore make a separate portfolio for your B&W images, and a different one for your color. Also, you would have a portfolio specific to portraits, and a separate one for "things" or whatever else you have.
At least this is the way I was taught in my photography classes.
However it also depends how you are making your portfolio. Are you actually making prints for a portfolio? Or making a gallery on a website? If you have a gallery on a website, that is very easy to just set up different galleries for each type of photography you do.