The question might seem silly but I never crafted so much (playing all 7 classes) and leveled up so fast (already paragon 925… it’s not the best but considering I seldom reach 800 in a season, 900 in 5 days is fair)…
Anyway I can farm ~1000 DP per hour, have lots of souls… but I am running out of blue, yellow and white!!! 
Where could I get lots of those?!
I recommend never upgrading yellows unless you have to, trying to gear up all classes and upgrading will drain your materials fast. You just get them by playing, rifts / bounties are the best. If you got more of one than the other you can also convert them in the cube
yea I am limiting upgrading the yellow… but I think I still upgrade yellow ring , amulet and weapons to get the rarer items… but yes good point!! 
Battlefields of Eternity is an abundant source of white materials.
Act V - Battlefields of Eternity. Hit up all the armor and weapon racks for white items to salvage into reusable parts. Convert them into arcane dust or veiled crystals as needed. Also Road to Corvus, Cathedral, etc…
Yeah, trying to upgrade all seven classes is like a nursing mother tryna’ feed triplets.
The “best” way to farm mats is to speed gr. It gives u paragon and u get gears, bloodshards which u give to kadala. Which u can salvage for mats. U will gain lots of yellows and FS. The yellows u then can trade for other mats by using the cube.
Is it the fastest way to get the mats? No. But your character grow stronger way faster than u spending time in bounties for mats.
However if u dont care about that bounties would be the best option simply since u not only get yellow and blues but bounty mats which could be used to reforge your gears. I dont recommend it though as u might spend hours on upgrading and reforging gears which might just drop for free in GR while u getting paragon
Also paragon increase strenght for all your characters so.
Edit: ethereals usually give yellow when salvaged. And they drop alot in GR if u can speed gr90+
Yellow items salvage into 1 veiled crystal. White items salvage into 6-9 reusable parts. Just saying…
Campaign mode.
The legendary drop rate seems to be reduced a great deal, but you could just run at Normal at specific places as mentioned and get a ton of white, blue, and yellows, pretty much guaranteed.
Or bounties.
Split bounties with a fast group are your best bet. Fast split bounties give very good whites/blues/yellows per hour.
Has to be splits though. If you’re running solo, it’s way too slow to be worth it.
Splits will give a bit of an excess of whites/blues, so you can convert to yellows to make up the difference. GRs give an excess of yellows though, so you can also just run a bunch of GRs after to even up if you don’t want to convert.
yeah just 400 from a bounty run! way to go!
Raid the weapon/armor racks of
good place to start.
bounties,lots of bounty bags
Either Battlefields of Eternity (as people have mentioned previously), or just regular rifts in general. I joined a level 12 snapshotted game to farm for low level ethereals a few days ago, and I was surprised at the number of blue/white/grey items in a single run.
best way is just split-farm bounties in 7-8 mins, you will get many whites and blues and then blast speed gr’s to get yellows from kadala
Puzzleitforeward community and run shared ancient puzzles. Go from 0 to 1000s farily quickly if you have the rings.
Salvage Ethereals. Lots of materials.
you can do bounties as well…you get 35 yellow, 90 white and 92 blue materials per cache. So if you do all acts in a group bounty run that’s 175 yellow, 450 white and 460 blue. So though you’re getting the other 2 materials faster you’ll get a lot of yellows from salvaging both from collecting and bloodshards.
If end up with like 5000 blue and 5000 white and 3000 yellow you can convert the other 2 to gain more yellow materials in the cube.
I did 1 hour of bounty, plenty of mat… (100 bounty craft mat available) and then… all gone in a matter of crafting too many yellow + hope of cain.
So I guess… for future reference…