You have to blow the cue off first, but they can be sealed with several products as mentioned by others here, If Your comfortable with using ca, then a thin Ca is good for sealing before sanding also, and can keep the darker colors from staining. What ever You use My advice would be to apply It By wiping lenthwise. The trick is not to goober the sealer up creating ridges and more sanding, but to wipe it on smoothly and evenly. The same trick can be applied to ring work to help prevent bleeding of 2 or more colors. Regardless of what You use, sealing the problem areas off after the taper is the best way I know of to prevent this. Even applying the base sealer for a base cote can cause colors to bleed, so that's Why pre-sealing them before the epoxy cote can be a good idea. You have to make sure that the direction you pre-seal in is not going to drag one color over the other though, or It will still stain, and even worse may end up locked in. It's tricky, but can be done with a little thought as to the direction Your sealing in. Like with rings It would be the opposite just hold your paper towel or whatever you use to apply sealer over the lightest color that will stain and kick the lathe over. You don't want to move your hand though or you will still drag the darker color over the light color. You just want to seal the light color at first. By no means is this a seal cote but just a a method to prevent staining while your finish prepping the cue to keep any darker colors from getting down into the pores of a lighter color.
Greg