Mosaic adhesive generally dries brilliant white for a reason
These two pics are the same tile, both pics taken at same time with identical lighting conditions. Difference is one is placed on brilliant white surface the other, on a piece of black marble!
A lot of mosaic can be slightly translucent. So if u use a darker adhesive the true colour will not be reflected.
And personally, if conditions allow, I'll start my mosaic around eye level (that way the mosaic that immediately catch the eye are perfect) and hang them as opposed to bottom up. Working in horizontal rows of two. I find they maintain their shape better. Unless they are very heavy and sag in middle. But your setting out has to be pin point or it'll end in tears. Haha
We all have our own technique with mosaic, do it how u feel comfortable, that'd be my advice.