Mood Board supports the grouping of arbitrary assets much the same as you’d expect from a page layout application. Select a number of assets and use Edit: Group (Command+G) to group them together into a single selection set. You will notice that selection marquee display in different colors; this is dependent upon the content that would be selected as follows:

• Default System Highlight Color (e.g. Orange)
  Items that are not part of a group will display using the default system color (specified in System Preferences: General: Highlight Color)
• Inverted System Highlight Color (e.g. Blue)
  Items that have been grouped will display in the inverse color of the one specified in your system preferences.
• Averaged System Highlight Color (e.g. Olive)
  If a selection contains a mix of grouped and un-grouped items, a color that matches the averaged hue between the previous two options will be displayed.

To ungroup previously grouped items use Edit: Ungroup (Command+Shift+U) Items that have been grouped will recall their selection together, and translate as a single unit.