In 'One Piece' they are on opposite sides, so in fanfiction, seeing them interact in different scenarios is fascinating. For example, they could be forced to team up against a common enemy. Their different abilities, like Smoker's smoke - based powers and Ace's fire powers, can also lead to some really cool fight scenes and power combinations in the fanfictions.
One common theme is the development of their relationship. Given their professional dynamic in the One Piece world, fanfiction often explores how that might transition into a more personal connection.
One great 'One Piece Smoker x Ace fanfiction' is 'Smoker's Redemption'. It explores an alternate universe where Smoker has to team up with Ace for a mission and their relationship develops from there. Another one could be 'Ace and Smoker: An Unlikely Alliance' which delves into how their different personalities clash and then find common ground.
Finding good 'One Piece Smoker x Tashigi fanfiction' can be a bit of a hunt. You could try using specific search terms like 'Smoker x Tashigi relationship fanfiction' in addition to just the character names. Also, some Tumblr blogs might be dedicated to One Piece fanfiction and could potentially have some stories about this pairing.
The One Piece manga is not in color. It's presented in black and white, which allows for greater focus on the linework and details of the illustrations. Colorized versions might exist in some limited cases, but the majority of the manga is black and white.