How to create rainbow text?

I've been playing with parley for a bit and have been trying to make rainbow text. In reality I'd like to animate each visual item to have it fade in or something, like how NPCs talk in a video game. If I add a color span for each letter I get something like this:

You can see it loops through white, blue, purple, yellow and repeats. This works for the most part but some colours seem to skip in places, especially with Devanagari script at the end.

If I colour it per-glyph I get less skipped colours but it doesn't really 'feel' like each colour is attached to a single 'character' of text. It's a vague, nebulous term, but I would say that for the purposes of this effect I'd like to split up the text horizontally in to pieces, but at the end you can see two glyphs are combining to make one vertical cluster. This would look a little strange to be animated in, I'd think.

Colouring it per-cluster seems to look the most 'reasonable' to me. A better example is Hebrew:

Though one thing that is broken with this approach instead of giving each Unicode codepoint a span is that right to left text no longer gets coloured properly as iteration is done in visual order. Using spans of Unicode codepoints fixes this, maybe that's the better approach? But I would have to do breaking myself by grapheme cluster.

To me it seems like it would be a good idea to let glyph runs map to visual clusters somehow but maybe this is a design error? I feel a bit uncomfortable segmenting text with so many spans but maybe that's the correct approach?

Start by reproducing the reported per-letter, per-glyph, and per-cluster coloring examples, including the Devanagari and Hebrew cases. Compare the behavior of Unicode-codepoint spans with visual-order iteration, then define what mapping or API should be considered done for RTL text and combined glyph clusters.