You need to extract 2 seven-figure grid references from the rainbow
square.
There is no significance to the colour of the numbers - black shows
up better on yellow! 
The cache is an ammo box and contained, when placed, trading
items, pathtags, log and pencil and a travelbug.
[Puzzle] Guvf chmmyr qbrfa’g arprffnevyl sbyybj n fgenvtug yvar gb gur fbyhgvba. Nyfb, gurer'f ab tbyq ng gur raq bs guvf envaobj. Ohg lbh jvyy svaq n tbyq-yvxr pbybhe ng gur raq bs gur frdhrapr.
[Cache]Haqre uvturfg ebbgfgbpx