Here is a layout with stepped gradient and black gridlines.
GRDTHK adjusts the thickness of the gridlines
LatDiv adjust how many steps are in the gradient

Note the thick black line down the middle resulting from cutting through the
skewed longitudinals.

-tgq

//START
camera {
  location  <5,10,-10>
  right     x*image_width/image_height
  look_at   <0.0, 0.0,  0.0>
  angle 12
}

light_source {
  <30, 50, -40>
  rgb <1, 1, 1>}

plane {
  y, -1
  pigment {rgb 1}
}

//HSL to RGB
#declare tHSL=function(H,C){(H/360+C/3)-floor(H/360+C/3)}
#declare qHSL=function(H,S,L){select(L-1/2,L*(1+S),L+S-L*S)}
#declare pHSL=function(H,S,L){2*L-qHSL(H,S,L)}
#declare CHSL=function(H,S,L,C)
{
  select(
    -S
  ,//S>0
    select(
      tHSL(H,C)-1/6
    ,//tc<1/6
      pHSL(H,S,L)+2*(qHSL(H,S,L)-L)*6*tHSL(H,C)
    ,
      select(
        tHSL(H,C)-1/2
      ,//tc<1/2
        qHSL(H,S,L)
      ,
        select(
          tHSL(H,C)-2/3
        ,//tc<2/3
          pHSL(H,S,L)+2*(qHSL(H,S,L)-L)*6*(2/3-tHSL(H,C))
        ,//esle
          pHSL(H,S,L)
        )
      )
    )
  ,//S=0
    L
  )
}
#declare RHSL=function (H,S,L){CHSL(H,S,L,+1)}
#declare GHSL=function (H,S,L){CHSL(H,S,L, 0)}
#declare BHSL=function (H,S,L){CHSL(H,S,L,-1)}
//END HSL to RGB


#declare LatDiv=6;    //Number of Latitude divisions, used for long too
(degrees= 90/LatDiv)
#declare GRDTHK=0.01;  //Line thickness for 1 unit radius sphere

#declare FX=function(x){360/(LatDiv*4)*int(x*LatDiv*4+0.5)}
#declare FY=function(y){int(y*LatDiv*2+0.5)/(LatDiv*2)}
#declare FZ=function(y){int(y*LatDiv+0.5)/LatDiv}

#declare FR= function(x,y,z){RHSL(FX(x),FZ(z),FY(y))}
#declare FG= function(x,y,z){GHSL(FX(x),FZ(z),FY(y))}
#declare FB= function(x,y,z){BHSL(FX(x),FZ(z),FY(y))}

#declare HSL=
pigment{
  average
  pigment_map{
    [function{   FR(x,y,z)   } color_map{[0 rgb 0][1 rgb <3,0,0>]}]
    [function{   FG(x,y,z)   } color_map{[0 rgb 0][1 rgb <0,3,0>]}]
    [function{   FB(x,y,z)   } color_map{[0 rgb 0][1 rgb <0,0,3>]}]
  }
  warp{spherical}
}

#declare AA=sin(GRDTHK/2);

#declare LAT=
union{
  #local i=+90/LatDiv/2;#while(i<90)
    difference{
      cone{0,0,2*y,tan(radians(i))*2 translate -y/sin(radians(i))*AA}
      cone{0,0,2*y,tan(radians(i))*2 translate y/sin(radians(i))*AA}
    }
    difference{
      cone{0,0,-2*y,tan(radians(i))*2 translate y/sin(radians(i))*AA}
      cone{0,0,-2*y,tan(radians(i))*2 translate -y/sin(radians(i))*AA}
    }
  #local i=i+90/(LatDiv);#end
}

#declare LONG=
union{
  #local i=+90/LatDiv/2;#while(i<180)
    cylinder{-z*sin(GRDTHK/2),z*sin(GRDTHK/2),1.1 rotate y*i}
  #local i=i+90/(LatDiv);#end
}

#declare PLat=
pigment{
  object{
    LAT
    pigment{onion scale 1/LatDiv
      pigment_map{
        [0.5-AA*LatDiv HSL scale LatDiv]
        [0.5-AA*LatDiv rgb 0]
        [0.5+AA*LatDiv rgb 0]
        [0.5+AA*LatDiv HSL scale LatDiv]
      }
    }
    pigment{rgb 0}
  }
}

#declare PGrid=
pigment{
  object{
    LONG
    pigment{PLat}
    pigment{rgb 0}
  }
}

difference{
  sphere{0,1}
  box{0,<1,1,-1>}
  pigment{PGrid}
  finish{ambient 0 diffuse 0.5}
}
//END

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