Just to update this in case anyone else needs a work around
Create a small .png graphics file and set page size to very small say 100x50 pixels fill the page with colour of you choice and ensure Opacity is set to say 30% then save the file e.g. LineColour.png in my case.
In your FPPDF module
// Add the image and store for future use
iPicIndex := oPdf.Images.AddFromFile('c:\myprog\art\LineColour.png', False);
// Use FPPDF function to paint the line (you can stretch by using whatever width and height you need.
DrawImage(fXpos, fYpos, fWidth, fHeight, iPicIndex);
Not the best solution as inflates the size of the PDF but it works.