Please follow below.
1. In the load script, Add below codes
LEt vColorLight=rgb(176,242,242);
LEt vColorMiddle=rgb(153,204,255);
LEt vColorMixDark=rgb(40,115,204);
2. Create a var chart object.
3. Go to Appearance > Colors > Disable the auto > Choose « By Expression » > Add below expression.
ColorMix2((rank(total Column(1))/(noofrows(TOTAL)/2))-1,$(vColorMixDark), $(vColorLight),$(vColorMiddle))