The article already contains most information about how to do that:
Color Banners
Your banner.txt files can now include ANSI color codes. You can do crazy things like this:
If you look at the Spring docs, you can then find more information about how to tweak the banner.txt file.
So, by adding a banner.txt file to the classpath, you can add your own banner. The background of a tile can be changed with ${AnsiBackground.NAME} where NAME is the name of the color. The list of colors can be found in the AnsiColor class.
Below the nyan cat you can see a small text with :: Meow :: Running Spring Boot 1.3.0.RC1 :: \รถ/, to create such text you can use the ${spring-boot.version} placeholder.
You can also create a banner programmatically with SpringApplication.setBanner(), but like I already set, it’s all in the docs.
The source of the actual Nyan cat banner can be found on GitHub, as mentioned by Stephane.