I bought Thompsons Waterproofing Solid Latex Stain in Cedartone and when applying to my current deck with over 130 spindles, the colour is too light.
It is currently I would say Honey Gold Colour stain that was used.
Please help - can I add another colour stain to this to darken it somehow?
With the solid stain lines of Sherwin Williams, MAB and all they are tintable so are easy to darken (so long as you do not need to go to a color that requires a different base). I don't know if Thompson's can be tinted or not but ask the paint store or check the website. You should have no problem mixing different colors of the same product together if that works to get the hue you want. The problem with Thompson products is the waterproofing is wax base which makes it difficult to use them.