Graphics @ 
     {Arrowheads[{{.1, 1, {Graphics[{Opacity[.1, Black], 
        Polygon[{{-1, 1/2}, {0, 0}, {-1, -1/2}}]}], 1}}}], 
      AbsoluteThickness[5], 
      Arrow[Line[{{2.5, 2.5}, {5, 5}}, 
        VertexColors -> {Opacity[1, Black], Opacity[.1, Black]}]]}

[![enter image description here][1]][1]

    Graphics@{EdgeForm[{AbsoluteThickness[10], Green}], FaceForm[], Rectangle[{3.9, 3.9}], 
      Arrowheads[{{.1, 1, {Graphics[{EdgeForm[], Opacity[.1, Black], 
            Polygon[{{-1, 1/2}, {0, 0}, {-1, -1/2}}]}], 1}}}], 
      AbsoluteThickness[10], 
      Arrow[Line[{{2.5, 2.5}, {5, 5}}, 
         VertexColors -> {Opacity[1, Black], Opacity[.1, Black]}]]}

[![enter image description here][2]][2]

Replace `Opacity[1, Black]` with `Opacity[1, Purple]` and `Opacity[.1, Black]` with `Opacity[.3, Orange]` to get

[![enter image description here][3]][3]

Alternatively, without `Opacity`:

    Graphics@{EdgeForm[{AbsoluteThickness[10], Green}], FaceForm[], Rectangle[{3.9, 3.9}], 
      Arrowheads[{{.1, 1, {Graphics[{EdgeForm[], GrayLevel[.9], 
            Polygon[{{-1, 1/2}, {0, 0}, {-1, -1/2}}]}], 1}}}], 
      AbsoluteThickness[10], 
      Arrow[Line[{{2.5, 2.5}, {5, 5}},
         VertexColors -> {GrayLevel[0], GrayLevel[.9]}]]}

[![enter image description here][4]][4]

Replace `GrayLevel[0]` with `Purple` and `GrayLevel[.9]` with `Lighter[Orange, .8]`  to get:

[![enter image description here][5]][5]


  [1]: https://i.sstatic.net/zVUCJ.png
  [2]: https://i.sstatic.net/Qe2vz.png
  [3]: https://i.sstatic.net/HMRyF.png
  [4]: https://i.sstatic.net/XgNFh.png
  [5]: https://i.sstatic.net/RCpY2.png