OK, yes, then you would need to switch to using virtual pins and coded control routines on the device.
Strip intensity control must come from the matched reduction of the PWM values across the channels, NOT the supply voltage.
Are you aware the zeRGBa already incorporates this intensity control via it’s vertical placement of the touch point? - How add brightness to zeRGBa?
And for example of virtual pin control of a PWM digital pin… this is a very simple, single channel, PWM pin control from a slider on V0 set with a range of 0-1024
BLYNK_WRITE(V0) // This functionRuns every time LED slider widget is adjusted
{
analogWrite(PWMpin, param.asInt()); // Send slider value (storred in param.asINT() )to real LED.
}
http://docs.blynk.cc/#widgets-controllers-slider
http://docs.blynk.cc/#blynk-firmware-virtual-pins-control-blynk_writevpin