I've been painting with Citadel paints since 1999 and haven't used anything else since then. I have to buy paints online but I don't know which paints to get.

I'm going to be buying reaper paints but their colors are different from citadel as far as naming is concerned.

I know there exists a Citadel to Vallejo and Reaper to Vallejo conversion chart but it is in spanish and I do not understand that moonspeak.

Basically I'm just going to buy a triad of paints to cover my Tau army, plus what ever paints usually come in the GW starter kit. Also, a few tubs of Black, White, some inks, and a few empty tubs.

My tau army will basically be what it is in the codex, Scorched Brown - Vomit Brown - Bleached Bone, thats gonna be the collar scheme. I was thinking either that or a deep red/maroon color but I'm just not sure which would look best.

After a short search with google I found this on the Reaper message board. Since I am not sure whether I am allowed to post a link to the board, just the list I found there... All credits go to Vaitalla.

Skull White: Pure White 9039
Chaos Black: Pure Black 9037
Scab Red: Mahogany Brown 9070 is closest (browner) or 9001 Red Brick (purpler). A mix of these would probably be the best of both worlds and match really, really close.
Red Gore: Deep Red 9002 is close
Blood Red: Clear Red 9094
Blazing Orange: Phoenix Red 9005
Fiery Orange: Clear Orange 9130
Golden Yellow: Marigold Yellow 9007
Sunlight Yellow: Sunlight Yellow 9008
Bad Moon Yellow: Clear Yellow 9095
Bilious Green: Spring Green 9146
Scorpion Green: Moth Green 9145
Goblin Green: Field Green 9167 is close
Snot Green: Clear Green 9096
Dark Angels Green: Pine Green 9010 is close, or mix Swamp Green 9175 with some Clear Green 9096.
Camo Green: Camoflage Green 9177
Catachan Green: Military Green 9176
Scaly Green/Jade Green etc.: The Reptilian Greens Triad 9184/9185/9186
Hawk Turquoise: Marine Teal 9077 is close
Ice Blue: Mix Clear Blue 9097 with Pure White 9039 to match
Lightning Blue: Sky Blue 9018 is close
Enchanted Blue: Cyan Blue 9117 or Sapphire Blue 9016 are close
Ultramarines Blue: Ultramarine Highlight 9189
Storm Blue: Ritterlich Blue 9115
Midnight Blue: Ultramarine Shadow 9187
Regal Blue: Brilliant Blue 9116
Nauseating Blue: Deep Amethyst 9079
Tentacle Pink: Close would be Pale Violet 9027 with a drop of white added
Warlock Purple: Clear Magenta 9098
Liche Purple: Clear Plum 9132
Imperial Purple: Clear Purple 9099
Codex Grey: Rainy Grey 9038
Shadow Grey: Twilight Blue 9020
Space Wolves Grey: Snow Shadow 9021 is close
Ghostly Grey: Ghost White 9063
Bleached Bone: Yellowed Bone 9143
Bubonic Brown: Tanned Leather 9031
Vomit Brown: Burnt Orange 9111; Leather Brown 9030 if you want it more
brown
Beastial Brown: Intense Brown 9138
Scorched Brown: Blackened Brown 9137
Snakebite Leather: Half-orc Highlight 9204
Leprous Brown: Saffron Sunset 9182
Vermin Brown: Chestnut Brown 9071 is close; a 50/50 mix of this and 9072
Rust Brown is close to perfect
Rotting Flesh: Bloodless Skin 9150 is very close
Elf Flesh: Rosy Highlight 9069
Bronzed Flesh: This color is very orange, and alarmingly close to Burnt
Orange 9111; however, I prefer a more natural skintone, 9044 Tanned
Skin, as a substitute for this one. If you want more warmth, mix 50/50 Tanned Skin 9044 with Golden
Shadow 9091.
Dwarf Flesh: Rosy Shadow 9067
Dark Flesh: Dark Skin 9041 would be closest. Add a drop of Ruddy Leather 9109 to deepen the color.
Burnished Gold: New Gold 9051
Shining Gold: Antique Gold 9050
Dwarf Bronze: Ancient Bronze 9049
Brazen Brass: Tarnished Brass 9198
Tin Bitz: Scorched Metal 9125
Mithril Silver: Polished Silver 9054
Chainmail: Honed Steel 9053
Boltgun Metal: Shadowed Steel 9052

Newer or previously neglected GW Colors:

Graveyard Earth: Uniform Brown 9127 is close, as is a 50/50 mix of 9161 Shield Brown and 9162 Driftwood Brown (though the latter is a little more grey)
Kommando Khaki: Terran Khaki triad is close
Fortress Grey: Misty Grey 9090 or Icy Grey 9174 may be close
Terracotta: Terracotta Clay 9170 is close
Desert Yellow: Dune Shadow 9154 is closest
Tanned Flesh: Fired Clay 9171 is closest

Hekau Chasut has given you the most common conversion chart (the only one I've seen actually). The only thing to keep in mind is that they won't match up perfictly.

If you have started your Tau army in GW colours you may have some difficulties switching them over to Reaper without an obvious difference in colour. I had the same problem when I switched over, so I would suggest you ither finish them in GW colours, or re-paint the GW painted models.