STRING* HitBoneName = str_create("");
///
var dist = c_trace(whatever);
if(dist>0)   //make sure something WAS hit
{  if(hit.entity!=NULL);     //make sure we have an ENTITY
   {  ent_bonename(hit.entity, HitBoneName, hit.vertex);    //get the bone name
      if(str_len(HitBoneName)>0)   //make sure a BONE has been hit
      {  //"HitBoneName" is the name of the bone we've hit.
         //and if you name them simple like L_Arm_0, L_Arm_1, R_Arm_0, R_Arm_1, etc 
         //they'll be easy to spot with
         if(str_cmpni(HitBoneName,"L_Arm_")==1)
         {   //some part of LEFT ARM was hit      }
         else if(str_cmpni(HitBoneName,"R_Arm_")==1)
         {   //some part of RIGHT ARM was hit      }
         else if(str_cmpni(HitBoneName,"L_Leg_")==1)
         {   //some part of LEFT LEG was hit      }
         else if(str_cmpni(HitBoneName,"R_Leg_")==1)
         {   //some part of RIGHT LEG was hit      }
      }
      else
      {
         //entity has been hit but no bone is assigned to that vertex
      }
   }
   else
   {
      //there was a hit but NOT an entity
   }
}
else
{
   //NOTHING was hit
}

Check before here that we have a sucessful hit on an entity.
1> var ClosestVertex = hitvertex;    //get the nearest vertex to the hit position
2> var UV[3];  vec_for_uv(UV, you, ClosestVertex);    //get the X,Y co-ordinate of that vertex in the UV map
3> BMAP* ColorMap = ent_getskin(you, 3);  //get the color-map skin, assume limb-color skin to be number 3
   var BmapType = bmap_lock(ColorMap);   //required to allow use of pixel_for_bmap
   var LimbPixel = pixel_for_bmap(ColorMap, UV[0], UV[1]);   //get the pixel from the colormap at UV position
   var LimbColor[3];   pixel_to_vec(LimbColor, NULL, BmapType, LimbPixel);    //convert pixel to a color vector
   bmap_unlock(ColorMap);   //free bmap now we are done with it