I recommend that you do a test piece (or test a bit of the piece that's there) and carve the foam some to remove the skin, which is probably what is reflecting and making it look off-color (and you're right! That a TOTALLY different color!). With the skin removed, the foam will absorb water and shouldn't reflect the light that way.
Now, here is the issue with this. Most expanding foams are petroleum based. With the proper cure time, I have personally used expanding foam underwater with no leeching, but it's possible that your's might. So, if you try this, don't stick it back in your tank to check it. Flush it with water in the sink or tub, then put it underwater in something you can see through to see if it is still that grey color. If the color is good, carve the foam on the log to the shape you want, flush it again, then soak it overnight or so and then put it in your aquarium. I think that carving it will also make the shape look more natural.