I say this with all honesty..... I don't see the Riviera as "gray" in color. Maybe my eyes process color differently. But I see the exterior as off-white, and the interiors are a more brilliant white. The whole resort feels bright and airy. It's what my wife hates about the Jambo and Poly lobbies -- dark and cavernous. While the RIV has a bright, airy, casual elegance.
And it's odd that you would compare the rooms to a "Midwestern downtown Hyatt" -- BY FAR, the Riviera has the most elegant rooms of any on-site property at Disney, surpassing the Grand Floridian easily. (that might change with the GFV renovation).
GFV studio:
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RIV studio:
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GFV 1 bedroom:
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Riviera 1 bedroom:
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Maybe it's me... but it's the Grand Floridian rooms that look like they are from a cheap chain hotel, while the Riviera rooms actually exude some finesse.