ha, now it works.
BLYNK_WRITE(V1)
{
int shift = param.asInt();
for (int i = 0; i < strip.numPixels(); i++)
{
strip.setPixelColor(i, Wheel(shift & 255));
// OR: strip.setPixelColor(i, Wheel(((i * 256 / strip.numPixels()) + shift) & 255));
}
strip.show();
}
void loop()
{
Blynk.run();
}