Allright, I've changed the color to a more reddish palette for the collisionable Tiles.
Heads up: my experience on the subject is mostly from playing games and some research.
While your graphics are good as is, I feel you can improve a lot with some techniques and ideas. In regards to the background, there are ways to create more contrast:
-Lower saturation for the background: making the background "greyer" helps it contrast with the foreground.
-Darker tones overall (see the boss rush room in Shovel Knight)
-Use a complimentary color for the background: in Castlevania 1, there are orange tiles against the predominantely dark blue background, that increases the contrast between them. From a quick google search:
Most browns are a kind of dark and muted orange (orange being a secondary colour - mixed red and yellow) and the complementary would therefore be a blue. A reddish brown's complementary colour will be a green, a purplish brown's complementary will be a yellow.
One technique that can help your graphics overall is
hue-shifting. Look into the subject, that can give more life to your graphics.