Our muscles are gray, and since the metal is already colored, why color it?
Real muscles are grey? Really? I mean, I know we usually see them when they still have some blood in them so they look red, but even without blood I wouldn't imagine them as actually being grey. Source?
This is one thing the movies should be taken as canon on, as it was very intentional to have them be the portrayal of the "bio in bionicle". So, it would seem it's "gray unless otherwise 'painted' or elementally colored".
As for the texture, I would guess that the "brain" texture is actually lungs (outer surface), while the others are all muscle. The more "wirey" one is just the same as the other one you called muscle-like but stronger for a Toa than a Matoran.
Yeah but they messed up other bits of canon (The Rahkshi bodies and Vahki had organic components even though they should be mechanical, except for the Rahkshi Kraata), so...
Any ideas on what may make flesh "elementally coloured", or purple? And why are we assuming grey is the default? I actually usually imagined them as having purple flesh, but I got conflicted when I would draw a bionicle and purple just went horribly in their colour palette and I'd think "what if it could be another colour...wait...what would it actually be?" and I wondered. I guess it could be any colour, if their flesh had a pigment in it. Or, if they could be bothered, they could of course paint themselves.
I guess that those lung-brains in the Nuva chests could make them pretty vulnerable. I mean, I might be a soft squishy measly human thing, but at least I've got a rib cage around my most vital organs - it would feel a bit vulnerable if my organs were partly exposed like that. Meh, I guess they just trying to stay true to the sets (their gears).
And if you look, the default on Mata Nui seemed to be purple:
Turaga Vakama: http://biosector01.c...raga_Vakama.png
The Toa seemed to be the only exceptions (and Rahkshi lol), and only some of the time (Tahu Nuva). However, on Metru Nui, everyone seemed to usually be grey again, except for the general populous of Matoran, who were the exact same models as from Mata Nui, so I don't think they should be counted. Instead, I think we should count the only "Metruvian" Matoran: the Toa Metru as Matoran. With them they were grey, but had abdominal muscles that was their primary colour:
Vakama's abdomen is red, not grey or purple: http://biosector01.c...oran_Vakama.PNG
Same principle with Matau: http://biosector01.c...toran_Matau.PNG
And here's Toa Vakama (dat pose tho): http://biosector01.c...etru_Vakama.PNG
There were some exceptions that were Metruvian, meaning I can't ignore them. Toa Nokama's flesh was purple, and Nuju's was blue. I think this was just done because it looked nice colour wise, and it certainly gives us something to think about. Since I don't think the Metru would have had time to paint themselves, it must be pigment, I.e. their actual flesh colour.
Toa Nokama (I always thought it was funny how she looks a little disgusted; maybe Matau had just done something?): http://biosector01.c...etru_Nokama.png
Toa Nuju: http://biosector01.c..._Great_Disk.PNG
Note in the set advert he seems to have white flesh just in between his shoulder joints (made so his shoulders could twist): http://biosector01.c...u_Metru_Set.png
Yeah...I don't think this gets us very far. It seems they can almost be whatever really, so long as it's either purple, grey, or some colour relating to their element, which is barely a rule, just a faint pattern ><
BTW, I noticed a while back that the Metru Matoran have smooth abdominal muscles, but when on Mata Nui they have built abs. I think this variation was kinda done to show that on Mata Nui all the Matoran had to do more physical work, so they all got pretty built, even the female Matoran, which i really respect Lego for doing.