Some nutrients can be translocated in the plant, some cannot. Here’s the breakdown, in order of their prevalence in the plant. I have placed the ones where deficiencies are rare in parenthesis.
N, K, Mg, P, (Cl, Na, Mo, Ni). These can be moved around so the plant will translocate them to newer leaves and shoots where they are needed. Therefore, when these are deficient older leaves will yellow first.
Ca, (S), Fe, B, Mn, Zn, (Cu). The plant cannot translocate these nutrients, they must remain where they are. Therefore, when these are deficient the newer leaves and shoots will yellow first.
Tip: Note that the macros (N, P, K) are all among the mobile group. This means that if you have discoloration at the growing shoots, you can almost always rely on a good micronutrient product to fix it.
Of the following: Si, Cl, Fe, Cu, Mo, Ni, Se, and Na, you simply will not see deficiencies of silicon (Si), chlorine (Cl), selenium (Se) or sodium (Na). Sulfur (S) deficiency is almost as rare, occurring only in very sandy soils with extremely low amounts of composted matter, and affects the younger leaves. Iron (Fe) is a fairly common, but easily spotted deficiency that also corrects quickly. Iron deficiency appears as a well defined striping of the leaves. Copper (Cu), molybdenum (Mo), and nickel (Ni) deficiencies happen but are rare. If I saw one of these deficiencies in my grow op, I would call friends in to have a look, that’s how rare they are. I am not going to bother with them. I think many authors include them in their books in the interest of completeness or maybe just to sell books, but including them just introduces possibilities in the grower’s mind that are very unlikely to really occur. You’re better off not being the proverbial hammer out looking for a nail.
Common leaf color changes due to deficiency of nutrient:
Chlorotic leaves, green veins
In order of likelihood, this could be boron (B), iron (Fe), magnesium (Mg), manganese (Mn), nickel (Ni), or zinc (Zn). Boron is a fairly common deficiency. It can be identified by the black necrosis of younger leaves, starting at the proximal end of the leaf. A little boric acid or borax in the water is all you need. This can be purchased at a hardware store. Iron, magnesium and manganese are difficult to distinguish from one another. Your best bet is to treat for all of them by adding a bit of commercial micronutrient to the water you are using. You will recognize zinc by the deformed, puckered leaves eventually accompanied by necrotic spots. If you want to keep it cheap, try adding a few iron bolts or nails and a few pennies to your bucket along with some epsom salts (1 tsp/gallon) and see if that works. That covers iron, zinc and magnesium, respectively. That leaves us with manganese and nickel. Manganese deficiency is often accompanied by necrotic spots. Manganese deficiency is often an indicator of high pH. Perhaps you over-limed? Drop the pH a bit. Nickel deficiency will appear as leaf tip necrosis and can be fixed by adding a commercial micronutrient product. As mentioned above, it is unlikely that you have a nickel deficiency.
Uniformly pale leaves (no green veins)
In order of likelihood, this could be either nitrogen (N) or sulfur (S). Hint: It’s not sulfur. Add some manure or other grow formula.
Leaf color changes that may be due to nutrient deficiency:
Spots on leaves, no green veins
A bit trickier, the order of likely problems is that your ppm is too high, you need to pull the ppm back a bit, you’re burning because your ppm is too high, buddy, or, if you are in soil, your pH is probably too low (especially if it happens when the plants are flowering). If your pH is OK, you need to add phosphorus (P) or molybdenum (Mo). Which? If your leaves are dark green (maybe even purple) and spotting, add phosphorus (P), if they are pale and spotting, add molybdenum (Mo) (in other words, a micro product). Note that a molybdenum deficiency can bring about a nitrogen deficiency in soils that are old and fully cycled.
Leaves curling, spotting, then necrosing
If in hydro, check that the ppm is not too high. If in soil, a common problem is nitrogen toxicity. If your leaves are curling inward and downward like eagles’ claws, you have over-fed. This is more common in soil than hydro because soil growers often cannot resist adding bottled nutrient to their soil (which already contains nutrient). I’m not sure why this is (after all, the point of growing in soil is to not have to do this), but I see it frequently. Check that your pH is in a healthy range (~5.5- 6.5) and if both of the above check out, add calcium, because that’s probably what it is.
At this point the odds of your problem having not been fixed is slim to none. As a rule, and as you may have noticed: 1) check your ppm; 2) check your pH; 3) add nutrient as indicated above. Make all adjustments slowly and with patience. Observe carefully for subtle improvements. An attempt to correct these problems quickly will cause more problems. Take it slowly.