To use as many colors as possible within the limits, the RGB values of your image palettes have to be compatible with the 16bit standard (5 bits for Red, 6 for Green, 5 for Blue) or else they won't be displayed ingame the same way they should look.
The following basic grey gradient is not the best example possible, but it might give you an idea of it.
★ The top one is what RPG_RT will display.
★ The bottom one is the real image.
There are unofficial patch files available for some versions that change how RPG_RT displays the game screen to 32bit method (though 8bit images are still needed they can be pushed to the limits way easier). But by applying you may get in trouble distributing your game.