diff --git a/src/app/features/galaxy-system/star-readouts.spec.ts b/src/app/features/galaxy-system/star-readouts.spec.ts
index ddcf87f..e514e77 100644
--- a/src/app/features/galaxy-system/star-readouts.spec.ts
+++ b/src/app/features/galaxy-system/star-readouts.spec.ts
@@ -88,7 +88,13 @@ describe('starReadouts', () => {
       derived: true
     });
     expect(value(starReadouts(PLACED_BY_GAIA, { radiusSolar: 0.141, radiusDerived: false, temperatureK: 2900, luminositySolar: null, luminosityDerived: true }), 'Radius')).toBe('0.141 solar radii');
-    expect(value(starReadouts(HYG_STAR, { radiusSolar: 584.3, radiusDerived: true, temperatureK: 3590, luminositySolar: null, luminosityDerived: true }), 'Radius')).toBe('~580 solar radii, from colour and brightness');
+    // A giant's temperature is its type's whatever its colour, and so is a dwarf's with no colour.
+    const betelgeuse = { ...HYG_STAR, name: 'Betelgeuse', spectralType: 'M1-M2Ia-Iab', colorIndex: 1.85 };
+    expect(value(starReadouts(betelgeuse, { radiusSolar: 584.3, radiusDerived: true, temperatureK: 3590, luminositySolar: null, luminosityDerived: true }), 'Radius')).toBe('~580 solar radii, from its type and brightness');
+    const gj3655 = { ...HYG_STAR, name: 'GJ 3655', spectralType: 'M8', colorIndex: null, colorSystem: undefined };
+    expect(value(starReadouts(gj3655, { radiusSolar: 0.106, radiusDerived: true, temperatureK: 2570, luminositySolar: null, luminosityDerived: true }), 'Radius')).toBe('~0.11 solar radii, from its type and brightness');
+    const kepler445 = { ...HYG_STAR, spectralType: 'M4', colorIndex: 1.66, colorFromTemperature: true, source: 'exoplanet-archive' };
+    expect(value(starReadouts(kepler445, { radiusSolar: 0.21, radiusDerived: true, temperatureK: 3157, luminositySolar: null, luminosityDerived: true }), 'Radius')).toBe('~0.21 solar radii, from its temperature and brightness');
     expect(value(starReadouts(HYG_STAR, { radiusSolar: 1, radiusDerived: false, temperatureK: 5772, luminositySolar: null, luminosityDerived: true }), 'Radius')).toBe('1.00 solar radii');
     expect(starReadouts(HYG_STAR, { radiusSolar: null, radiusDerived: true, temperatureK: null, luminositySolar: null, luminosityDerived: true }).some((readout) => readout.label === 'Radius')).toBe(false);
   });
diff --git a/src/app/features/galaxy-system/star-readouts.ts b/src/app/features/galaxy-system/star-readouts.ts
index ddd638a..93ba09e 100644
--- a/src/app/features/galaxy-system/star-readouts.ts
+++ b/src/app/features/galaxy-system/star-readouts.ts
@@ -1,4 +1,5 @@
 import { spectralClassification } from '../../shared/astro/spectral';
+import { temperatureFromColour } from '../../shared/astro/stellar';
 import { formatDistance, formatLuminosity } from '../../shared/format/quantity';
 import { StarRecord, SUN_STAR_ID } from '../../shared/models/star.model';
 import { StarSurface } from '../body-detail/body-view-model';
@@ -58,17 +59,30 @@ export function starReadouts(star: StarRecord, surface?: StarSurface): HudReadou
     ...(surface?.luminositySolar
       ? [{ label: 'Luminosity', value: formatLuminosity(surface.luminositySolar), ...(surface.luminosityDerived ? { derived: true } : {}) }]
       : []),
-    ...(surface?.radiusSolar ? [radiusReadout(surface.radiusSolar, surface.radiusDerived)] : []),
+    ...(surface?.radiusSolar ? [radiusReadout(surface.radiusSolar, surface.radiusDerived, radiusBasis(star))] : []),
     { label: 'Source', value: catalogue === 'HYG' && star.distanceFromGaia ? 'HYG, Gaia DR3 distance' : catalogue }
   ];
 }
 
+/**
+ * What a derived radius is worked out from besides the brightness: the temperature the star's
+ * colour gives, or its type's — a giant's always, and a dwarf's with no colour the table reads.
+ * 11 546 radii read "from colour" whose temperature no colour went into: 10 702 giants with one, and
+ * 844 stars with none, GJ 3655 (M8) among them.
+ */
+function radiusBasis(star: StarRecord): string {
+  if (star.colorFromTemperature) {
+    return 'its temperature';
+  }
+  return temperatureFromColour(star) ? 'colour' : 'its type';
+}
+
 /** Two figures for a derived radius, three for a published one: 0.105 is not what colour gives. */
-function radiusReadout(radiusSolar: number, derived: boolean): HudReadout {
+function radiusReadout(radiusSolar: number, derived: boolean, basis: string): HudReadout {
   const digits = derived ? 2 : 3;
   const figure = radiusSolar.toLocaleString('en-GB', { minimumSignificantDigits: digits, maximumSignificantDigits: digits });
   return derived
-    ? { label: 'Radius', value: `~${figure} solar radii, from colour and brightness`, derived: true }
+    ? { label: 'Radius', value: `~${figure} solar radii, from ${basis} and brightness`, derived: true }
     : { label: 'Radius', value: `${figure} solar radii` };
 }
 
diff --git a/src/app/shared/astro/stellar.ts b/src/app/shared/astro/stellar.ts
index 1f7383b..dcc5707 100644
--- a/src/app/shared/astro/stellar.ts
+++ b/src/app/shared/astro/stellar.ts
@@ -151,7 +151,7 @@ export function luminositySolar(star: StellarPhotometry): number | null {
  * off-table colour is more often a bad one than an extreme star — HD 49748, G5 V, at B−V −0.32 —
  * and the type is read instead.
  */
-function sequenceAtColour(star: StellarPhotometry): DwarfSequencePoint | null {
+function sequenceAtColour(star: Pick<StellarPhotometry, 'spectralType' | 'colorIndex' | 'colorSystem'>): DwarfSequencePoint | null {
   if (star.colorIndex == null) {
     return null;
   }
@@ -175,6 +175,15 @@ export function effectiveTemperatureK(star: StellarPhotometry): number | null {
   return giantSurface(star.spectralType)?.temperatureK ?? (sequenceAtColour(star) ?? dwarfSequenceAtType(star.spectralType))?.temperatureK ?? null;
 }
 
+/**
+ * Whether {@link effectiveTemperatureK} reads the star off its colour rather than off its type: not
+ * for a giant, nor for a star with no colour the table reads, which since 206e88a is 821 dwarfs
+ * placed at their type's row. The card said their radii came "from colour and brightness".
+ */
+export function temperatureFromColour(star: Pick<StellarPhotometry, 'spectralType' | 'colorIndex' | 'colorSystem'>): boolean {
+  return giantSurface(star.spectralType) === null && sequenceAtColour(star) !== null;
+}
+
 /**
  * What a giant's type says of its surface: its effective temperature, and the bolometric
  * correction the dwarf sequence has at that temperature — both off the type, so that the two a