It depends on your connection. If you're using a component connection, you may have one or more of the RGB (red, green, blue) wires crossed. Check that, and also try a different cable, if you have one. I've used audio and composite video cables (audio - red, white, composite - yellow) to make component connections in a pinch. I seem to recall getting a B/W picture in the past when I either used bad cables, or connected one of them wrong.
Thank you for contacting us here on the answer forum. I would like to be of some assistance to solving why you only see a black and white picture. Ill post an article that has the most common reasons for the Slingbox giving a black and white picture.
Hope this helps,
Sling Moderation Team