A mixing apparatus having a mixing impeller affixed to the end of a mixer shaft which is reciprocably and rotatably or non-rotatably mounted thru mixer seal structure which has surface portions adapted to be brought into sealing engagement with the open filler end of a retail off-the-shelf tube of viscous caulking or sealant compound or the like, wherein the shaft and impeller are adapted to be reciprocated thru the compound and an additive contained in the tube substantially the entire length of the tube to rapidly and intimately mix the compound with colorant or other additive added directly into the retail tube thru its filler (back) end.