You will find it very difficult because the veneer is very thin and water will have penetrated probably right through. Strip all the coating off the panel with either a hot air gun or chemical stripper - don't sand at this stage. You may have to experiment because you don't know what the coating is.
Then use diluted bleach first, but you may well find that does little and oxalid acid is the next strongest solution if you are lucky you might get the panel back to a fairly even colour, but you can always use some coloured woodstain such as Colron to get an even colour before varnishing with a matt or silk varnish such as Ronseal. Gloss highlights imperfections so best avoided!
The alternative is to re-veneer the whole panel or even paint it a flat white set off with the varnished trim. Difficult to know what would look best without seeing the whole interior plus knowing your skills and your tastes in decor!