GIMP :: How To Shade One Edge To From Red To Purple To Indigo

Say you have a irregular object that is in shades of red and you want to shade one one edge to from red to purple to indigo. You select specific red pixels and replace them with blues. You brush in the amount, area, and shape that you want to change without changing and other colours. You then select another shade of red pixel and used a darker shade of blue and brush over part of the area that you have already done to darken it. You repeat as necessary to get the effect you want.

I have tried masking and color select but the edges are too defined and not giving the effect that I want. Most of my less powerful graphics programs have this tool but unfortunately they can no longer handle the graphics I am working with.