Minecraft Java Edition Commands – How to Use /setblock to Place a Colored Bed in Minecraft 1.12

In Minecraft, an exact integer coordinate (like 10, 64, -5) is actually on the corner between 8 different blocks. A center of a block is at 10.5, 64.5, -4.5.

When using a command such as /fill or /setblock, if a whole number is given, Minecraft automatically adds 0.5 to all of the numbers. This is because blocks must be aligned to the grid properly, so it must assume that when a user says "10" they have actually just truncated "10.5"

This isn't a programming mistake or glitch, it's because it literally can't place the block at an exact coordinate of "10".

On the F3 screen in newer versions, the "Looking at" coordinate will tell you what you actually have to type in to set the block you are looking at.

This answer was written prior to the release of 1.13 and "The Flattening" that came with it. An updated answer can be found here.

You're actually pretty close, and definitely on the right track. Instead of {Inventory:[{id:<block ID>,data:<data value>}]}, use {Inventory:[{id:<block ID>,Damage:<data value>s}]}. The s after the data value is important since it's stored as a short, not as an int. (As an aside, if it were stored as a byte, you would append a b instead.)