If you’re going to do it properly you’d actually attach candles to your tree, although this is slightly riskier (I’ve been woken and run out of the house by my mum in the past while my dad attempted to put out a flaming branch).

What's the problem with coloring the lens?

LEDs emit a limited number of wavelengths of light.

Incandescent lamps emit a broad spectrum of light.

Filters (colored lens) can knock out certain colors, allowing others to pass. Filters cannot add colors that aren't there to begin with.

You can't just filter a monochromatic light into another color. Filtering LEDs doesn't work as straightforwardly as your incandescent-based intuition would lead you to believe.

Width of the emitted spectrum.