Prismatic Wall RAW says:
(...) when a creature attempts to reach into or pass through the wall (...) as it passes or reaches through each layer, the creature must make a DEX save or be affected by that layer's properties (...)
Standing still inside the wall (it is 1 inch thick) is not an attempt to reach into or pass through. The creature indeed passed through when it entered the wall.
So, by RAW, does it have to make the DEX saves if it chooses to stay inside the wall after entering it?