UKC

Are 190 weight merino tights warm enough under a pair of hardshell pants in about the  0 to -10 range 

Not quite sure what 190 weight is, light/heavy?, but I regularly use merino long johns under my hardshell salopettes and its fine. I've got some thick ones (fancy Norwegian Devold ones - although actually they were in a bargain bin in a Finnish sports shop) and some thinner ones (Aldi specials), and to be honest it doesn't seem to make a massive difference which ones I wear, they work fine. Because I normally drive quite a long way before going out winter climbing I normally have some merino or synthetic boxers on as well, as I'll only put the long johns on before suiting up and leaving the car. So I guess I have one extra layer of insulation between my round the vitals!

Think 190 is mid weight ish not sure on how good merino tights are for warmth though

I've got 200 g/m2 merino from Icebreaker and 200 g/m2 merino from Aclima. For 0 to -10 C, the Icebreaker is too thin , and the Aclima is fine.

I use 180 gsm wool bottoms down to about -20. You may get cold but not uncomfortable, fine if you are moving and have good upper layers, decent gloves, warm socks etc.