Can we use Kanji for「すみれ」? I want to describe violet color. I've tried 「菫色」to look here, but no luck..
It appears so.
For example, it's recognised in this aggregate dictionary.
Japanese Wikipedia also has a page for it.
And searching the Shonagon corpus yields 15 counts of 菫色; 28 counts of すみれ色; and one count of すみれいろ。This indicates that perhaps the kana variant すみれ is more common, though in literary instances, the kanji variant is also acceptable. (Although, admittedly, 15 and 28 are too low a count to do any meaningful statistical analysis.)