Since Gen 3—Pokémon Ruby & Sapphire—each Pokémon type was given a color to symbolize it and help make it stand out against the other types. The colors used were a bit plain, and some types (like Dragon and Poison) had two colors... but they worked.

Later as the games grew more sophisticated, the colors for the types grew and changed slightly... with actually the most significant color changes occurring in Gen 8 (Sword/Shield); THOSE colors can be seen below.

Anyways, I was bored one night, so I went through each game and pulled the various colors that have been used to define it. The results are in the pixelized image just to the upper-right; I started with Gen 8 then worked backwards, with each type taking up two pixel rows, and each Gen in two pixel columns. I won't even begin to explain what the colors actually come from—each game had different ways to utilize each color and shade—but you can trust that I did the most accurate job I could.

For the pièce de résistance, I took each color and blended them together, thus forming the most overall and average and succinct definition of that type. Those colors are in the right-most column in the graphic above, but I've reproduced those colors and their hex color codes below. I thought about doing the same for every other color, but I'm too lazy to do that, so you can just download the image and use the eye-dropper tool in Photoshop to grab it.

To wrap things up: I like doing this kind of stuff because it's fun and I'm just curious to know what color totally summarizes that type. But more importantly, as an artist, knowing these colors definitely helps with my art, because it gives me a pallet to work with. You'll definitely see some examples when I start drawing art for the GS Space World fake card set... but imagine a Ground-Water Pokémon drawn and colored using only the colors that were used for that type across 15 years of Pokémon games. I can't wait to see what that could look like! :D

TYPE COLOR NOTES
Normal #ACAD99
Fighting #C45D4C
Flying #90AAD7
Poison #B369AF
Ground #CEB250
Rock #BAA85E
Bug #ACC23E
Ghost #816DB6 Turns out to be too much like Dragon
Steel #9FA9AF
Fire #E87A3D
Water #639CE4
Grass #82C95B
Electric #E7C536
Psychic #E96C95
Ice #81CFD7
Dragon #8572C8 Too much like Ghost
Dark #79726B
Fairy #E8B0EB

And there we are!

...

Now although I'm gonna stick with NOT covering all the colors used in EVERY game, I did say that I worked out the colors used in Pokémon Sword/Shield. That info is covered below.

You can see an example of what color goes where in the blank images below; the top is an example of the Pokémon's type info, while the bottom is an example of the Pokémon's attack. The white circle represents where the type symbol would go, while both images are colored in the three shades.

And so the below chart is the HTML code for all three colors for each of the 18 types, as well as an example of what those colors would look like within the type and attack graphics in the game. If other types show up—like the "???" type, which was programmed into the game as a fail-safe, going as far as programming a ???-type plate for Arceus—I'll be sure to add it to the list. ... Heck, I might actually make an entry for both ???-type and Bird-type just for the hell of it.

SYMBOL TYPE NAME ATTACK
:SWB Bug #91c12c #4d553e #add080
:SXB
Bug
:LXBBug Attack
:SWD Dark #5a5467 #44454a #908d96
:SXD
Dark
:LXDDark Attack
:SWN Dragon #0c6cc4 #374957 #7a99cf
:SXN
Dragon
:LXNDragon Attack
:SWL Electric #f4d43b #5d5a42 #f6dc85
:SXL
Electric
:LXLElectric Attack
:SWY Fairy #ed91e3 #5c4e5d #f2adec
:SXY
Fairy
:LXYFairy Attack
:SWF Fighting #ce4267 #594149 #d98698
:SXF
Fighting
:LXFFighting Attack
:SWR Fire #ff9d54 #605043 #ffb68d
:SXR
Fire
:LXRFire Attack
:SWV Flying #90a9dc #4d515a #aebde4
:SXV
Flying
:LXVFlying Attack
:SWH Ghost #526aad #424953 #8e97be
:SXH
Ghost
:LXHGhost Attack
:SWG Grass #63bc5a #445547 #94cc8f
:SXG
Grass
:LXGGrass Attack
:SWA Ground #d97846 #5b4940 #e19f87
:SXA
Ground
:LXAGround Attack
:SWI Ice #73d0bd #465858 #9cdacd
:SXI
Ice
:LXIIce Attack
:SWC Normal #909ca2 #4c504f #acb4b7
:SXC
Normal
:LXCNormal Attack
:SWO Poison #a96ac9 #524858 #be98d5
:SXO
Poison
:LXOPoison Attack
:SWP Psychic #fb717b #5e484a #fa9c9d
:SXP
Psychic
:LXPPsychic Attack
:SWK Rock #c6b88a #58554e #d3c8a8
:SXK
Rock
:LXKRock Attack
:SWM Steel #5a8fa3 #444d52 #90acb8
:SXM
Steel
:LXMSteel Attack
:SWW Water #4e92d2 #424e5a #8cade0
:SXW
Water
:LXWWater Attack

If anything else comes up, I'll be sure to share it here!