RGB LED Strip without microcontroller
I have successfully done that very thing in the past to replace a broken light bulb in my car for a fraction of the price.

It probably depends on the specific strip and LEDs, for me the color was not a "perfect" white, as expected, but close enough.

If you are worried about it working or not, and have some LED to spare, just cut one LED off the strip and run a test on it. Then you can proceed with some peace of mind to the whole strip.

I hope this helps.