Custom Graphics seem to have limited range

I fly a lot around Lake Ponchartrain and New Orleans. The Mississippi River and Lake Ponchartrain are both generally brown, muddy waters. Using the default graphics with AS2012, Lake Ponchartrain looks like the turquois waters around Key West, so I figured it was not treated as "inland" but rather "tropical" water by AS (probably - I assume - because FSX classifies it as such). No problem - I merely customized the theme and recolored my selected tropical water choice. It worked - to a point. It appears that the water changes from my recolored brown to the AS2012's turquois blue about 5.2 NM from the airport - making me think that the custom-colored graphics have a limited range before the default AS2012 graphics kick in. I tried redefining the application range for wx but the brown water changes to blue at the same point regardless.Interestingly, I recolored the inland waters to a greenish-brown, but the Mississippi River shows up as purplish, and maintains that color well beyond the range at which the tropical water fails to maintain it's recolorization. In fact, as far as I know, if I recolor the inland waters, there is no limited diameter to the application of the recoloring, unlike the tropical.Is there a work-around to this? It seems like I need to be better educated about how the graphics work?