Your video is 128×128, but the NES screen is 256×240, so the picture sits in the middle with a border around it. The NES fills that surrounding area with tile 0 — and tile 0 is also the top-left 8×8 corner of your image. They are the same tile, so whatever is in that corner becomes the border pattern, repeated across the whole frame.

Unticked: the border is a tiled repeat of your image's top-left corner. Sometimes that looks deliberate, and if the corner happens to be flat it looks fine.

Ticked: that one tile is blanked, so the border becomes a clean solid colour. The cost is the top-left 8×8 pixels of the video, which go solid too. On most footage you will not notice; on something with detail in that corner you will.

The border colour is the darkest of your four colours. The palette is sorted by brightness during conversion, and the blanked tile takes the first entry — so on dark footage the border tends to disappear, and on bright footage it reads as a dark frame.

Preview NES Output converts the first frame only and draws it in real NES colours. It is the quick way to check your scaling and colour reduction before committing to a full conversion — a 64-frame run takes a while, and squashed proportions or a muddy 4-colour reduction show up immediately in frame one.

Preview also fills in the thumbnails in the Palettes section, so run it once before going in there or the swatches have no picture to show.

The preview always uses palette 1, whatever you have selected below. The other seven are shown as thumbnails in the Palettes section.

The ROM carries 8 palettes of 4 colours, and all 8 are baked in when you convert. On the NES you cycle through them live with B — so these are performance presets, not one single look you have to settle on.

Within each palette the four colours run darkest to brightest. Conversion sorts your GIF's colours by brightness, so slot 1 replaces the darkest tone in the picture and slot 4 the lightest. Keeping that order roughly intact keeps the image readable; deliberately scrambling it is where the more lurid false-colour looks come from.

Each palette shows a thumbnail of frame 0 rendered in its own colours, so you can compare all 8 at a glance. Run Preview first to populate them.

Load colours from ROM pulls all 8 palettes back out of a NESFlix ROM you made earlier and drops them into the editor — handy for reusing a look across clips without setting 32 colours by hand. It only accepts MMC3 ROMs built by this tool; anything else is rejected, since the colours are read from a fixed position that only holds palette data in these ROMs.

It boots in four-up mode. These are the original NESFlix ROM's controls, from NO CARRIER & Batsly Adams — the converter only supplies the frames and colours, it does not change how the player behaves.