What is the difference between color guard and honor guard?

Depending on how far back you go, they are the same thing - color guard has its origins in the military, literally being there to guard the "colors" (aka the flag). Over time it evolved, and you have the modern color guard activity associated with Marching Bands and winterguards. However, as far as the military is concerned, it IS still called color guard - honor guard is a synonym, most likely used to avoid confusion. For example, at winterguard competitions, there is an honor guard to present the colors.