Magical force is transparent and colorless. But sometimes visible.
The paradox is that sometimes force energy is invisible, but sometimes it isn't:
Wall of Force creates an invisible effect.
Mordenkainen's Sword, which creates a "sword shaped plane of force" that is clearly visible. It makes normal attacks (without advantage) so we can conclude it's not an "unseen attacker."
Visible Yet Transparent
The solution to this paradox is that force is transparent and colorless, yet often visible, just just as water and glass can be. In physical terms, the force-filled air is a denser medium than normal air.
Shape and Light
It's the shape that makes the difference: a wall of force will look like a perfectly clear, unreflective sheet of glass. In other words, it's quite invisible.
A sword-shaped, or disk-shaped area of force can interact with light like glass or water. Edges will be apparent, and objects seen through them will be distorted, like viewing a pencil in a glass of water. The edge can look like the surface of water, or perhaps more like a halocline.
A sword shape is a well-recognized danger, so all observers will quickly be aware of the threat.
Confusion about Tenser's Floating Disk
In the case of Tenser's Floating Disk the case is a little more subtle. Adventurers familiar with them won't have trouble spotting or keeping track of one. But the shape is less obtrusive than something like a sword, and they might go unnoticed. This has caused a lot of conflicting eye-witness accounts over the years about whether the disk is "visible" or not.
(Example: If peasants witness a wizard standing on one, they are more likely to report a wizard hovering in mid-air than a wizard standing on a transparent disk.)
Concentrated Force Energy
Magic Missle creates glowing manifestation of force energy. This is a much more concentrated form of force, which is evident because mere contact with this energy damages most creatures.