Why does steak change colour after seasoning?

Details on beef color changes can be found here.

Briefly:

• When beef is first cut (or ground or whatever), it has a purplish hue, caused by deoxymyoglobin, a pigment that can only exist in the absence of oxygen.
• When exposed to oxygen, a chemical reaction occurs, producing oxymyoglobin, which leads to the bright cherry red color most people associate with "fresh meat."
• Eventually, the molecule oxydizes, producing metmyoglobin, which can no longer bind oxygen -- and this turns the meat brown. (There are natural enzymes present which can convert back to oxymyoglobin when in the presence of sufficient oxygen, though these will eventually be depleted. Packaging can preserve the strong oxygen-rich environment for longer, which is why grocery store meat stays bright red on the surface.)
• When meat is cooked, the globin denatures and forms a tan/gray hemichrome pigment which can no longer convert back to the other myoglobin pigments.

Anyhow, this is all a preface to explain that certain chemical additives will change the chemistry here, pushing the reactions to move to or away from certain pigment states.

The primary one that you mentioned is salt. As discussed in the linked article:

If salt has been incorporated into patties or is present in the case of enhanced steaks, salt decreases metmyoglobin reducing activity allowing more MMb to accumulate [i.e., turning brown]. . . . In addition, salt promotes heat denaturation, or breakdown, of myoglobin [i.e., turning tan or gray].

Basically, salt causes the pigmentation molecules to convert faster to states resembling old or cooked meat, causing the color change you see.

This is off-topic, but an important point to note!

From a purely culinary point of view, it all comes down to when you season. For example with proteins (Meats) it is highly recommended to season just before the cooking process. The simple answer to this been that seasoning agents such as salts and acids draw moisture out of the product. It is important to retain this moisture(the flavor) while the meat is been seared.

Try this with two identical cuts, one seasoned before hand(say 15 mins), the other just before searing. Use two pans that have similar properties.You will notice the cut of meat seasoned before hand will have more moisture in the pan while it is been seared. You want to be able to maintain a dry and hot cooking surface to maximize the caramelization process for optimum flavor.

On a side note, use minimum oil as possible, next to nothing, let the fats from the meat do the work. i.e use a oil quantity relevant to the fat content of the meat.

This does not apply to marinated techniques. You need to add the seasoning before hand for best results.

Final, this also applies to vegetables. For example, if you are doing a stir fry and you want the vegetable cooked but crispy, season the dish at the end of the cooking process. You get better results in terms of texture and color.

Once again sorry for the off-topic answer, thought I will share as we sometimes forget how important seasoning is, using it and when to.