"Organic" dyes:

White: Lily of the valley, generates in forests, can be farmed in flower forest.

Light Gray: Azure Bluet, Oxeye Daisy, or white tulip. Plains, sunflower plains, flower forest.

Red: Poppy, Red Tulip, Rose Bush, Beetroot. Poppy spawns anywhere grass spawns. This and yellow will be the most basic, as Dandelion does the same thing.

Orange: Orange Tulip. Slightly less basic than yellow and red by virtue of being craftable by their combination.

Yellow: Dandelion or sunflower. Dandelion spawns just about anywhere grass spawns.

Green: Cactus. Very slow to farm, probably worth more.

Lime: Sea pickle. Unsure how rare that is.

Light Blue: Blue Orchid. Only spawns in swamps.

Blue: Cornflower. Grows where light gray grows.

Cyan: Blue+Green

Purple:Blue+Red

Magenta:Allium, Lilac, various combinations of others. Allium only grows in flower forests. Lilac can only be found through worldgen in forests and flower fields.

Pink: Pink Tulip or Peony. Peony only grows through worldgen. Remember that tulips come in a variety of colors, so tulip exclusives are rarer.

Brown: Cocoa beans. Very hard to get IMO.

Inorganic dyes:

Black: Ink sacs. I do want black honey, but I'll specifically use a wither rose for that: It's endgame materials.

Grey: Only achievable with black.

White: Tempted to make white flavor require the specific flower as well