Palettes and Colour on the Next

Synopsis

Every video layer that comes after this chapter — LoRes, Layer 2, Tilemap, Sprites — uses the same palette mechanism. The Next has eight 256-entry palettes (ULA “first” and “second”, Layer 2 “first” and “second”, Sprites “first” and “second”, Tilemap “first” and “second”), each entry is 9 bits wide (RRRGGGBBB plus an optional priority bit), and they all funnel through the same two NextRegs. We cover it once, here, and every later chapter just says “set up its palette as in Palettes”.

Topics:

• The 9-bit colour space. RRR-GGG-BBB layout, the “extra blue bit” trick (NextReg $44), and why 9 bits gives you 512 colours instead of 8.
• The palette banks. All eight, what each one feeds, and why “first” vs “second” exists (live-swap without redrawing).
• Selecting a palette to edit. NextReg $43 — bits selecting which of the eight palettes is the current target for the index/value registers.
• Writing palette entries. NextReg $40 (palette index, auto-increment), $41 (8-bit colour value), and $44 (the 9th bit + priority bit). The auto-increment lets you upload a whole 256-entry palette in one tight loop.
• Per-pixel priority. The priority bit on a per-entry basis, and how it interacts with the layer compositing rules covered later in Compositing.
• The transparency colour. NextReg $14 (global) and $4A (sprite/tilemap-specific) — pixel values that get treated as see-through.
• ULA palette mode. Switching the ULA from its hardwired colours to a programmable palette via NextReg $43.

What you should know first

• I/O and NextRegs.
• The ULA Screen and Border — gives you a “before” picture so the palette swap exercise is visually striking.

What this chapter unlocks

• Every chapter in Part V from here on.

Planned exercises

• Palette shift. Take a ULA screen and rotate its 16 palette entries every frame — animated colour without touching screen memory.
• 512-colour gradient. Fill the entire palette with a smooth gradient and display it on Layer 2 (uses Layer 2).
• Palette swap. Build the same scene twice — once in palette “first”, once in palette “second” — and toggle between them on key press to demonstrate instant theme switching.