Current limitations of a MOSFET
You may see that the mosfet won't turn on all the way. The datasheet shows that it's Voltage Gate - Source Threshold (VGS(TH)) is 3V typical, 4V max. Meaning that it may barely work at the RPI'S 3.3V level. That Mosfet is not well suited for 3.3V logic level applications. At 3.3V, the FET won't even pass fractions of an Amp.
The RPI's 3.3V logic isn't turning the Mosfet on all the way.
If you want to stick to that mosfet, you could use a simple NPN transistor as a driver. A 2n3904 or 2n2222 or any similar. It's not critical, it's just a switch. Logic will be reversed, so on the RPi, A logic High/3.3V will turn the leds OFF.



simulate this circuit &ndash; Schematic created using CircuitLab
The RPI's ground and the 12V supply's ground need to be tied together as well. If anything, make sure you have that happening right now as that's a common mistake. It could be as simple as not having connected it right.
Otherwise, you want a &quot;Logic Level Mosfet&quot;, one that has a VGS voltage of 3.3V.
That said, depending on how long your led strips are, an 1 Amp supply may not be enough. Make sure the fully on White (All 3 channels) current is under that amount.