Quoted By:
Red/Green = RG
Blue = Blue (standalone), RGB (with Red and Green), or RB (as international Red and Blue)
Pikachu = Pikachu (standalone/Japanese), Yellow (standalone/English), RGBP (technical Gen I), RGBY (usual Gen I abbreviation), RBY (as international Gen I)
Gold/Silver = GS
Crystal = Crystal (standalone) or GSC (with Gold and Silver)
Ruby/Sapphire = RS
FireRed/LeafGreen = FRLG
Emerald = Emerald (standalone) or RSE (with Ruby and Sapphire)
Diamond/Pearl = DP
Platinum = Platinum (standalone) or DPPt (with Diamond and Pearl)
HeartGold/SoulSilver = HGSS
Black/White = BW
Black 2/White 2 = B2W2
X/Y = XY
Omega Ruby/Alpha Sapphire = ORAS
Sun/Moon = SM
Ultra Sun/Ultra Moon = USUM
I will simply say Generation I/Gen I in place of Red, Green, Blue, and Pikachu/Yellow, or RGBP/RGBY/RBY, and Generation II/Gen II in place of Gold, Silver, and Crystal, or GSC. Generations with remakes in them are a little more vague because something like HGSS can be seen as a testament to both Generation II and IV.
I say Pikachu instead of Yellow because the game is literally called "Pocket Monsters Pikachu" in Japan and "Pokémon Yellow Version - Special Pikachu Edition" (often just "Yellow" for brevity) in North America. It's not a common thing to do in the English fandom, but I feel that it's the accurate choice, and it's evidence that only Red, Green, and Blue were intended to be named after colors. The Japanese names "Kin" (金) and "Gin" (銀) (Gold and Silver) specifically refer to the metals, not the colors.
By the logic I'm using, one might wonder why I don't call Red "Aka" (赤), Green "Midori" (緑), Blue "Ao" (青), Gold "Kin" (金), and Silver "Gin" (銀) since they all use kanji for their titles, but their Japanese title screens say "Red Version", "Green Version", "Blue Version", "Gold Version" (on the Super Game Boy), and "Silver Version" (on the Super Game Boy) in English respectively.
