best thing to do would be to move it slightly or possibly un interlock it. It happens sometimes and it is mainly a certain area of the map or odd angle that makes it like that. If that doesn't work then either you can try another object or move the whole structure. hope that helps
It happens because of light cast upon the map in certain areas. This effect is common in the skybubble (except in the bubble the opposite effect happens and scenery glows bright yellow). Try interlocking it like megathumbs suggested, or use a different object. Wood bridges and scaffolding don't seem to be affected by the light, so use them is necessary.
Just save and quit or delete and replace the object. I've had that problem before, once I had a block turn nearly totally black but when I reloaded the map it was normal colour.