I need to darken new sandstone sills and mullions.Is feasible to paint on a mix of mortar dye.?

If you are trying to age sandstone, I would suggest, find a lime supplier near you then get some lythomix, plus a small amount of yellow powder (from the same place) you dry mix the 2 components together to achieve the desired colour. Be careful with the powder as you only want a very small amount, colour will darken as you are mixing it. We use this method, and can nearly get exact match of stone. The mortar dyes will fade over a short time.

i use cold tea or cold coffee,try on small sample area first.

I have used Horse manure and water, it ages the stone a treat.

Worked with some guys patching up pebble dash and they used, used cooking oil and mixed in some plain old soot to blacken it? They painted the areas with a brush and the next day U could hadly tell the diff? They mensioned that by the time it fades natural weathering would have taken place and U would hardly notice the differance? As above try a little area and see..