Overlay images can be set in Skin Settings > Fanart. You can do some interesting things with this like vignettes, lighting or textured overlays (the silhouette.png also works well).
You can create a colour scheme by copying one of the colour xml files in the skin.eminence/colors folder. Give it a new name and it will be selectable in Appearance settings. You only need to include the definitions you want to change, any missing colour definitions will be pulled in from the defaults.xml file. The colours are in ARGB Hex format, so the first two characters are the alpha channel. Post one or two screenshots with your xml to give an idea of what it looks like. Any impressive colour schemes I will include in future updates on the repo.
Key for main colours
This is the blue highlight focus colour used through the skin in views and hubs etc.
This is a stronger version of the highlight colour. This is used for text focus (e.g. settings level change). Generally this matches the posterhighlight colour, just with more opacity to make it stronger.
This is the colour of the background. The alpha channel needs to be full opaque (FF)
These are the colours of unselected items. e.g. borders around posters, hub items, list panel etc. the NF version is used in settings items.
This is the colour of the header tile on the home screen or fullscreen video
This is the header icon and text colour on the home screen or fullscreen video
This is the colour of the header tile in library views etc.