Making background solid color How can I make the background color consist of only one color like the picture above?
Usually I made L shaped plane, applied bevel + smooth to make a background, as shown below

there is still a distinct border between the wall area (darker yellow) and the floor area (lighter yellow), but the first pink image does not have that
What kind of ways would you recommend?
This is less flexible than compositing, but if you're in Cycles, and lazy, you can, for example:

Split your world material into a mix between 2 colors, one to light the scene, and one to be visible to the camera. Here the scene is lit only by an environment HDRI, with its color tweaked though a Hue Saturation Value node.
Sit your object on a plane masked in its 'Object' tab &gt; 'Visibility' panel to be a shadow catcher:

For this kind of result..

There are quite a few possible approaches .. maybe 'Shadow Catcher' would be a fruitful search term.