I often start the class by having students grab a P-1000 or P-200, setting them to 500 and 200 µl respectively, and together slowly pressing down to find the first stop, then the second. We transfer a bit of water together, then I leave them to their own.
Students often recognize c1 * v1 = c2 * v2 but have never actually used it (or have forgotten how to use it) to actually make dilutions. The original protocol, from which this was adapted, has quite explicit instructions. I usually allow students to struggle, ask them to consult their colleagues, etc, and then when a group is properly stuck will help them through it.
Stand by the standards and compare students' results with them. Students often don't have a good idea of how close is "close enough." Both the volumes and colors should be precisely the same!
Comparing students' samples to standards is easier if you set the standards on a piece of white paper.
If a student's sample is substantially different, ask them to go back and try it again.