Question in the title. Currently, Mars is a small orange dot in the sky. But what if green, blue, and some white were added? I wouldn't expect it to be too different- a large portion of it will remain pretty much the way it was, but with more snow (Tharsis bulge is REALLY high up).
3 Answers
To terraform Mars, broadly speaking would make it more like an analogy of Earth. As you say, it might be a little colder, but we would expect a terraformed Mars to have oceans, snow and maybe some imported plant life.
How would Mars look from Earth? A good starting point would be to see it the opposite way around. Here is how Earth looks from Mars.
Like Earth from a distance, a terraformed Mars would likely appear as a pale blue or white dot. If you want more snow or ice than water, it would surely lean towards white rather than a deep blue.
Mars currently has albedo 0.250, 81.7% of Earth's somewhat brighter 0.3. A terraformed Mars would presumably have a similar albedo. That would make it slightly brighter and bluer, about 0.2 ($2.5\log_{10}(0.3/0.25)$) magnitudes brighter. But this is not going to change the overall magnitude ($\approx -3$ at most) much.
Various aspects of the terraforming assumptions may nudge this up (more ice, extra light from solar mirrors) or down (less clouds? deliberate ice darkening?)
I pulled these pictures from Google. I hope it is enough to answer with pictures?
https://william-black.deviantart.com/art/Terraforming-Mars-392449850