There are three types of slider "styles" currently supported in RR:
1-Blank: The slider shows just the pointer, nothing behind it (just the off layer of the screen itself)
2-Single Image: The slider shows an image that is displayed UP TO the pointer position and the remainder shows whatever is on the off screen layer)
3-Multi Image: The slider shows one of 21 images based on the position of the pointer.
Based on your description, seems like you want to use "2", and have one type of color/drawing for the OFF layer of the screen (where the slider is) and have another color/drawing for the image that will be displayed up to the pointer position.
This is explained in Skin commands.txt, but basically you'd have:
IMAGE.PNG is the image used to fill the slider up to the pointer (Pointer.PNG) and anything else will be "empty" displaying whatever is on the OFF layer of the screen at the slider position.
If this isn't what you're looking for, please provide images to explain what you intend to do.