Either approach works; the former gives a unified appearence and is easier to hang straight, while the latter is easier to carry to and from the meeting. It is also possible, but often expensive, to have a commercial house reproduce your completed poster as a single large sheet of paper, which can then be rolled into a cylinder for transport.
The choice of a background (and complement) color is up to you. The general consensus, however, is that softer colors (pastels, greys) work best as a background - they are easiest to view for hours at a time, and offer the best contrast for text, graphic, and photographic elements. See the Use of Color page for some hints on choosing colors.
Check with your graphic arts department, or with local suppliers, for color samples. Plan on using 2 - 3 large sheets of mat board per poster; plan on 3 sheets of board if you anticipate making a few errors. Next, measure the boards and cut them to size, then lay them out within your taped boundries. Once you've made these decisions, the next step is to begin Trimming the elements of your poster. Or, you can RETURN to the Main Menu page.