Actually, I think that M4 looks pretty cool in two tone.
Most US troops during the Vietnam War probably would have given anything to have been issued an M16 with that colour scheme, because quite a lot of them got hold of green pvc/duct tape and made their M16s look exactly like yours. Although to be fair, a lot of that tape on their M16s was to stop the thing rattling and falling apart, since the first M16s weren't exactly built like tanks, but, you can find a lot of pics of M16s, AR15s, M4 etc looking a lot like yours, so if anyone doesn't like it or think it looks odd or incorrect, then they don't know as much as they think they know! It will certainly work better in a woodland skirmish in that colour scheme than an all black one would.
Here's a pic of an M16 with green electrical tape on it during the Vietnam War. Keep in mind that colours are very faded on that old 1960s Kodachrome picture, that tape would actually have been a pretty bright green, not dissimilar to the colour on your M4, because the soldier who stuck that tape on his rifle would have wanted the colour to look like foliage with bright sunlight on it. Also notice that in the same picture, lying on a pack, there appears to be another M16 variant (probably an AR15) in the background which has had a similar treatment with green tape:
http://s161.photobucket.com/user/toddmart68/media/walk3.jpg.html
A few things about that picture indicate that those guys know what they are doing incidentally. One of the weapons has a suppressor on it, the other appears to be an AR15, both of which were not typically issued to your average 'grunt' in Vietnam, but more often to units such as LRRP or special forces A teams. Note too that the weapon does not have a sling, which was done to avoid having stuff which could rattle or get hung up on branches. Both soldiers are wearing tiger stripe camouflage, as opposed to the standard M65 olive drab army gear you tend to see being worn in that war, also an indication that they are a more 'pro' unit than your average GI. They are in what appears to be a fire support base up in mountainous terrain, evidently high up enough to be in the cloud base, so they are probably near the Central Highlands. In 1968, if that date on the picture is correct, the location and the equipment seen would mean that they are probably involved in covert operations along the Western border of Vietnam.