As a non-native English speaker, I wouldn't be offended at all.
Typically for written forms that are drastically different(e.g. English vs Japanese) I don't think there's a need to put them in different colors. The whole point of using different colors is to easily identify the difference. I remember when I first studied English, when my teacher listed British written form side by side with the American written form, she'd put them in different colors. In which case, I think colors made sense.
If you must use a different color for Japanese, be sure to do some research on the color symbolisms in the Japanese culture. I have a hunch that you shouldn't use the color red. Even though red is a festival color in most Asian cultures, but it's rarely used for text.
For example, in China, writing people's names in red is almost taboo. This is because in the ancient time, the warden would write the names of the prisoners who are to be executed.