Here is the refined color palette with a shade that is darker than Royal Purple ( #7851A9 ) but lighter than Rebecca Purple ( #663399 ):

json
{
  "palette_name": "Adjusted Transition with Medium Purple First",
  "colors": [
    {
      "name": "Medium Purple",
      "hex_code": "
                            
                            #9370DB
                        "
    },
    {
      "name": "Amethyst",
      "hex_code": "
                            
                            #9966CC
                        "
    },
    {
      "name": "Royal Purple",
      "hex_code": "
                            
                            #7851A9
                        "
    },
    {
      "name": "Dark Orchid",
      "hex_code": "
                            
                            #9932CC
                        "
    },
    {
      "name": "Rebecca Purple",
      "hex_code": "
                            
                            #663399
                        "
    }
  ]
}

In this palette, Dark Orchid ( #9932CC ) serves as the fourth color, positioned between Royal Purple and Rebecca Purple, ensuring a smooth transition.

Would you like any further modifications or assistance with this palette?