What to use

It is common to use /blockdata for changing placed sign data. So, we will use it for now. The syntax of /blockdata is /blockdata [x] [y] [z] [dataInJSON]. Note that we use JSON formatting for text.

Basic for sign

To input a text with /blockdata, we will use Text1 tag. To write "Hello World" into the first line of the sign, simply just do /blockdata [x] [y] [z] {Text1:"{"text":"Hello World"}"}. The quotes is there for JSON formatting. If you need multiple line to display, use either Text2, Text3 or Text4.

Coloring

There is a way to change color of the sign using the color sub tag. There is a lot selection of color. Here is all of the available one:

• gold
• red
• dark_red
• yellow
• dark_purple
• light_purple
• aqua
• dark_aqua
• black
• white
• green
• dark_green
• blue
• dark_blue

For writing "Hello World" in aqua, you write

/blockdata [x] [y] [z] {Text1:"{"text":"Hello World","color":"aqua"}"}

into a command block. It will change the sign with the first line saying "Hello World" on aqua.

Bold

For bold text, you use bold sub-tag. This sub-tag is a boolean, meaning the value can be only true or false for example:

/blockdata [x] [y] [z] {Text1:"{"text":"Hello World","bold":true}"}

will give you a sign with text "Hello World" with bold.

The real answer

Type this into a command block changing [x], [y], and [z] into the coords of the sign:

/blockdata [x] [y] [z] {Text1:"{"text":"Place Block","color":"dark_blue","bold":true}"}

Full list for Raw JSON tags

You can use /give command too.

by using the BlockEntityTag You can place a sign down with text.

for example if I wanted a sign with "RedIsCool" with red color I would have to do

/give @p sign 1 0 {BlockEntityTag:{Text2:"{"text":"RedIsCool","color":"red"}"}}

in a command block, and then power it.

Version Update

1.12−

Above answers apply.

1.13

1.13 changed the command syntax. NBT now goes directly after the block/item ID, without a space. Here are some updated 1.13 commands from the previous answers:

/give @p sign{BlockEntityTag:{Text2:"{"text":"RedIsCool","color":"red"}"}} 1
/setblock ~ ~ ~ sign{Text2:"{"text":"RedIsCool","color":"red"}"}}

In addition, /blockdata is now a deprecated command, and has been replaced by /data merge block. Run the following command to change placed block data:

/data merge block ~ ~ ~ {Text1:"{"text":"RedIsCool","color":"red"}"}

1.14+

The 1.13 commands still work in 1.14+, but additional modifications are possible to make the commands easier to read:

• You can use /data modify instead of /data merge. This allows you to modify one specific tag at a time. This is more useful when performing complex operations with NBT tags, but isn't really needed for simple operations.

• 1.14 added support for single quotes ' to surround NBT string tags instead of double quotes ". Therefore, you can make the following replacement:

  {Text1:"{"text":"RedIsCool","color":"red"}"}
  {Text1:'{"text":"RedIsCool","color":"red"}'}
  

which will make it much easier to read.

It is example sign with custom text color:

/give @p oak_sign{BlockEntityTag:{Text1:'{"text":"Example","color":"#123456"}'}}

#123456 is your color hex. You can convert RGB to color hex with this website.

(RGB is Red Green Blue)