could be anything from a duff tuner , open resistor in the g2 bias area, RGB stage problem....get it to a tech for a diagnosis not based on internet guesswork. Bear in mind the age of the set, unless the tube is in very good shape and hasnt been used a whole lot, it may be worth getting another set.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix televisions.