There are a couple of ways you could approach this. First just take the output of a Textfield, draw it to a RenderTexture, then tint that by drawing it in a Quad and colouring the vertices.
The alternative would be to manipulate the Textfield’s internal Mesh data. The only problem is the data is handled in two ways, depending on the sort of TextField, i.e. whether its using TrueType or a bitmap font. So depending which you are doing you could add code to add a gradient to the appropriate compositor, by e.g. manipulating the quads that get added to its MeshBatch.