You can edit the file /etc/nanorc or the file ~/.nanorc to add some syntax for the files that you want.
You first need to specify a line with the name of the syntax and the files that you want to be highlighted by your syntax. Adding a line like :
syntax "c-file" "\.(c|h)$"
means that you want all the .c and .h files to be highlighted with the content that you specify after this line, with some regular expressions. For example, add the following line after the previous line to highlight all the integer numbers (positive or negative) in red :
color red "\<[-+]?([1-9][0-9]*|0)?\>"
This file text should be placed into a file named /etc/nanorc. If you are not root, then into ~/.nanorc.
It will enable colors for various type files.
color fgcolor,bgcolor regex
For the currently defined syntax, display all expressions matching the extended regular expression regex with foreground color fgcolor and background color bgcolor, at least one of which must be specified. Legal colors for foreground and background color are: white, black, red, blue, green, yellow, magenta, and cyan. You may use the prefix "bright" to force a stronger color highlight for the foreground. If your terminal supports transparency, not specifying a bgcolor tells nano to attempt to use a transparent background.