Tri wrote:standard army colours + lots of gold trim
Honestly, that's all there is to it most of the time. If you want to convert the marines, then you could swap out their legs for some of the Chaos Space Marine legs (or possibly some other bits also) so that you can give them gold trim.
Another option that you have is to paint the minis in a mix of gold and the chapter color.
For example, you could do the wristbands, elbow pads, shoulder trim and lower legs (below the knees) in gold, while preserving the rest of the mini in the chapter colors. Or a reversal of this would also work.
One thing I have noticed is that metallic colors tend to be less intrusive on a color scheme. You could even do most of the mini in gold or even silver, but if there is still a strong element of your chapter color, it will shine through. Look at some of the mixed color marines (Like the Mentor Legion) for ideas as to how you could break down your colors.
Another strong color to set the honor guard apart is white. The same as above, just add in some strong white elements to the mini, but keep a strong basis of your chapter color and you can pull it off nicely. This works very well for Dark Angels and Ultramarines. Some strong white elements do not detract from your chapters colors in the same way that metallic colors don't.