There's probably a better way than this(such as a Find/Replace type action), but you could Add a Layer that is Blue and with the Visibility set low(so it can be easily seen through).
edit: That'll probably look kinda wonky though. If you just want one part Blue, you could either:
1) create new Layer then Polygon Select(if PaintShop has that or calls it that) around what you want Blue, then colour it Blue
2) Do a Select by colour, then copy/move that selection to a new Layer, and colour Blue.