"keep it real" - chinese translation
The dictionary definition of "keep it real" is "be yourself" ; "don't be fake" 

"忠於(loyal to) 本色 (real color)"  means " be true to one's own nature" 

I do think "be yourself " could substitutes  "be true to your nature" 
These two phrases are somewhat comparable, so I would say it is not wrong to translate "keep it real" as "忠於本色"

However, 忠於本色 sounds quite awkward. It is not a common phrase, because 本色 means 'characteristic' . we generally see it in phrases that depict some unique characters.  For example 英雄本色，強人本色，奸雄本色. Placing a verb 忠於(loyal to) before 本色 just not sounds right in my ears.

Since it is a lyric, you can interpret more freely. For example: 


"忠于自我" (be true to yourself), 
"保持真我" (keep your true self) 
"別装假" (don't fake)