Generate on a key color in FLUX (ComfyUI)

One thing to know before anything else: in FLUX there is no working negative prompt. Everything has to go in the positive text, which is why the prompt below is written as description rather than as a ban list.

The prompt

The subject is a lavender sprig tied with twine. Lavender is a purple word, and a blue field behind violet petals is asking for a collision, so the sprig renders on pure red.

lavender sprig tied with twine, isolated on a completely flat, uniform, solid pure red (#FF0000) digital chroma-key background. The pure red background fills the image edge to edge like a flat digital chroma-key screen with no gradient, staying at full brightness inside every gap and opening in the subject; no reflection or tint of pure red on the subject. Every edge of the subject is crisp, sharp and hard against the pure red, with no soft, blurry, feathered or glowing transitions, no depth-of-field blur, no haze or halo; inside every hole and gap the pure red stays at full brightness right up to the edge. Shaded parts of the subject keep their own natural color, never a pure red tint. The entire subject is centered and completely inside the frame with at least 10% empty background margin on every side, nothing cropped or touching the image edges. No frame, no border, no paper, no mockup, no vignette, no text, no watermark, no deformed or duplicated parts. No floating or detached fragments, no stray specks, dust or debris anywhere on the background; every element is physically attached to the subject.

It goes into the positive CLIP Text Encode node of the standard ComfyUI workflow. FLUX pairs two text encoders, CLIP-L and T5, and T5 is the one that reads sentences, so paste the text in as written, punctuation and all.

If your graph still carries a negative CLIP Text Encode node from an SDXL template, leave it empty and stop maintaining it. The distilled FLUX checkpoints run at CFG 1, and at CFG 1 the negative side is never evaluated, so a carefully built ban list there does nothing at all.

If the cut says the key color is inside the subject

Rusty tones in the twine are the usual reason on this subject. The fallback is blue; violet petals sit far enough from #0000FF that the cut holds them apart.

lavender sprig tied with twine, isolated on a completely flat, uniform, solid pure blue (#0000FF) digital chroma-key background. The pure blue background fills the image edge to edge like a flat digital chroma-key screen with no gradient, staying at full brightness inside every gap and opening in the subject; no reflection or tint of pure blue on the subject. Every edge of the subject is crisp, sharp and hard against the pure blue, with no soft, blurry, feathered or glowing transitions, no depth-of-field blur, no haze or halo; inside every hole and gap the pure blue stays at full brightness right up to the edge. Shaded parts of the subject keep their own natural color, never a pure blue tint. The entire subject is centered and completely inside the frame with at least 10% empty background margin on every side, nothing cropped or touching the image edges. No frame, no border, no paper, no mockup, no vignette, no text, no watermark, no deformed or duplicated parts. No floating or detached fragments, no stray specks, dust or debris anywhere on the background; every element is physically attached to the subject.

A mixed flower bouquet rendered on a solid pure red background, beside its cut on a transparent checkerboard with holes where the bouquet's own reds matched the key — A red-key render carrying reds of its own. The cut loses those flowers, the badge reports it, and the blue version above is the fix.

Keep the sentences

Do not compress the block into tags. “flat background, chroma key, no gradient, 4k” is Stable Diffusion dialect; T5 was trained on prose and follows long plain sentences noticeably better than keyword lists. The prompt above runs well past a hundred words and FLUX tracks all of it. Shortening it to save tokens saves nothing here and costs the phrases that keep the gaps bright.

Guidance stays at 3.5

The FluxGuidance node defaults to 3.5, and that is where it should stay. Raising it feels like it ought to enforce the background harder; what it does instead is drag the key color into the subject as a tint, red-washed stems on a red key, and the badge then reports key color inside the subject on renders that would have been clean at 3.5. When a background comes out dim or wavy, change the seed or add steps rather than guidance.

What FLUX gets right

It takes “staying at full brightness inside every gap” literally. The enclosed spaces other models shade, the gap inside a twine bow or the slivers between stems, come out the same red as the border, so the cut leaves them properly empty instead of foggy.

Which key to pick, and what each sentence in the block is for, is on the guide.

Use it on your own subject

The prompt above is one subject. For anything else, keep your own wording and paste this block on the end of it.

Then cut them out

Save the batch as PNG at full size and drop it into ClipBrook. Your images stay on your computer. Nothing is uploaded. The cut measures the exact red your render used from its border, and each image comes back as a PNG with a transparent background.