What's wrong with saying 染眉毛 (rǎn méimao) to mean "color eyebrows" when 染头发 (rǎn tóufa) means "dye hair"? It's common for Western women to get their eyebrows tinted.  The process basically involves dying your eyebrows (although it varies from place to place).

I found it difficult to get this done in China.  I attempted to describe it as 染眉毛 (rǎn méimao) thinking it's the same as 染头发 (rǎn tóufa) literally "dye hair", but with "hair" replaced by "eyebrow".  (Jukuu.com examples: 染头发, 染眉毛.)

It seems that 染眉毛 is plain wrong, but I'm wondering why.

Question: What's wrong with saying 染眉毛 to mean "color eyebrows" when 染头发 means "dye hair"?

Google Translate gives 着色眉毛 (zhuósè méimáo) as the phrase to use, combining 着色 ("tint") with 眉毛 ("eyebrow").
I see nothing wrong with 染眉毛. You could also say 给眉毛染色 and everybody would understand. 

着色眉毛 sounds odd. The common way to say 着色 is 给...着色, and 给眉毛着色 for this case.