The yellow in Lego is really vivid. It's even intense for a Bart Simpson sculpture (someone made one). The closest thing to Astro's artificial skin is either Tan or Flesh. Flesh coloured Lego is really rare so I went with Tan.
This is actually enough Tan around (at least, over the course of this project). It's roughly double the cost of the common colours.
Here's the latest picture:

There's a lot to fix up - the yellow and black inside his chest needs to be replaced with dark grey (like the grey on the viewscreen). I also ran out of inverted slopes so the inside of the door is missing a lot of pieces.
So far so good. B)
I know you had already talked about it, but do you use a model or are you doing it all yourself? I'm amazed that you can open his chest!!! Congratulations! Just getting this far is impressive!
The Astroboy models I have are not particularly true to the anime, so I'm going by screenshots. It sounds harder than it is, because all I have to do is measure his dimensions on the screen and I know how big a Lego brick is.