example of how to build an instanced sphere while adding additional attributes to the mesh

setup.jsx

  // Builds instanced data for the packing
    const objdata = useMemo(() => {
        let sphere = new SphereGeometry(1, widthSegments, heightSegments);

        // build colors
        let palette = colors({
            numberOfValues: 20
        })

        const settings = {
            dimensions,
            packAttempts,
            maxCount: numInstances,
            minRadius,
            maxRadius,
            padding
        }

        const circles = pack(settings)
        let positions = new Float32Array(numInstances * 3)
        let scales = new Float32Array(numInstances)
        let colorsData = new Float32Array(numInstances * 3)

        let count = 0
        circles.forEach((circle, i) => {
            positions[count] = circle.position[0]
            positions[count + 1] = circle.position[1]
            positions[count + 2] = circle.position[2]

            scales[i] = circle.radius

            let index = Math.floor(Math.random() * palette.length)
            let color = palette[index]
            colorsData[count] = color[0]
            colorsData[count + 1] = color[1]
            colorsData[count + 2] = color[2]

            count += 3
        })

        let ipositions = new InstancedBufferAttribute(positions, 3)
        let iscales = new InstancedBufferAttribute(scales, 1)
        let icolor = new InstancedBufferAttribute(colorsData,3)

        return {
            index: sphere.index,
            attribs: {
                iPosition: ipositions,
                iScale: iscales,
                iColor:icolor,
                ...sphere.attributes
            }
        }
    }, [])

start.jsx

   <mesh}>
            <instancedBufferGeometry
                instanceCount={numInstances}
                index={objdata.index}
                attributes={objdata.attribs}/>
            <rawShaderMaterial
                ...<whatever your shader options are> 
                />
        </mesh>

Uses npm module pack-spheres to determine positions and radius of spheres.