Yes. Find the two pins where the voltage reads +12 volts.What do you mean? Negative and positive?
When I changed colour to blue 2nd pin has -12v and 4th has -12vIt was on red colour if that matter.
Ok but does the dimmer function works with this wayTying the three outputs together as you have shown, is a bad idea. You might create a short and damage the wifi controller.
Why not tie the B (blue) connection to the led strip, leaving the R and G disconnected? Then you just control the strip with the B channel, ignoring the other two. Blue and white LEDs use a similar amount of color (closer than red or green LEDs)
It looks like the Wi-Fi controller is for an RGB strip. It has three channels; one for each color. Just use one.