I must write 색 after a color?

I must write 색 after a color?
Example: I can say: 빨간색 사과 ? or only 빨간 사과 ?

No, you can skip 색.

I must write 색 after a color?

I must write 색 after a color?
Example: I can say: 빨간색 사과 ? or only 빨간 사과 ?

빨간 => Adjective.

            So, placing '빨간' before noun '사과' is very natural. 

빨간+색 => Noun.

                So, 빨간색 사과 is noun + noun. '빨간색 사과' is also natural

빨간색 사과 totally makes sense in Korean. But it somehow gives a feeling that you're emphasizing on the color '빨간색'. On the contraray, 빨간 사과 seems balanced between '빨간' and '사과'

In conclusion, You can use both words, which has no problem!

I must write 색 after a color?

I must write 색 after a color?
Example: I can say: 빨간색 사과 ? or only 빨간 사과 ?

si esta bien, existiria una diferencia en sentido pero es ignorable....sigues usandolo!!!