sqcdy_acf_rgb.php

<?php
/*
Plugin Name: Square Candy ACF RGB color output
Plugin URI: http://squarecandydesign.com
Description: provides a function to output RGB values from ACF colorpicker fields
Author: Peter Wise
Version: 0.1
Author URI: http://squarecandydesign.com
*/

/*

Example Usage:

h1 { color: rgb(<?php sqcdy_acf_rgb('my_colorpicker_field'); ?>); }

<?php $rgb = sqcdy_get_acf_rgb('my_colorpicker_option_field', 'option'); ?>
h2 { color: rgb(<?php echo $rgb ?>); }

div { background-color: rgba(<?php sqcdy_acf_rgb('my_colorpicker_field'); ?>,0.5); }

<?php echo 'h3 {color: rgba(' . sqcdy_get_acf_rgb('my_colorpicker_field') . ',0.5); }'; ?>

*/


// output an acf colorpicker field as R,G,B
function sqcdy_get_acf_rgb($field, $id) {
	if ( function_exists('get_field') ) {
		$hex = get_field($field, $id);
		// extra sanity check that we're dealing with the hex colors
		// in #NNNNNN format that ACF/WordPress/Iris colorpicker returns
		if (strlen($hex) == 7 && substr($hex,0,1) == '#') {
			$r = hexdec(substr($hex,1,2));
			$g = hexdec(substr($hex,3,2));
			$b = hexdec(substr($hex,5,2));
			return $r . ',' . $g . ',' . $b;
		}
		else {
			// provide a default in an emergency
			return "128,128,128" ;
		}
	}
}

function sqcdy_acf_rgb($field, $id) {
	echo sqcdy_get_acf_rgb($field, $id);
}

I would leave this plugin/code exactly as it is and then avoid get_sub_field() in your theme code. Just do get_field() on the repeater and then loop through the values.

See "Basic Loop (PHP foreach loop)" half way down this page instead of the "Basic Loop" example at the very top of the template section. https://www.advancedcustomfields.com/resources/repeater/

Nevermind - I see where the issue would come up from you code now. Maybe try keeping the conversion function separate from the ACF stuff like this:

function sqcdy_get_acf_rgb($hex) {
	// sanity check that we're dealing with the hex colors
	// in #NNNNNN format that ACF/WordPress/Iris colorpicker returns
	if (strlen($hex) == 7 && substr($hex,0,1) == '#') {
		$r = hexdec(substr($hex,1,2));
		$g = hexdec(substr($hex,3,2));
		$b = hexdec(substr($hex,5,2));
		return $r . ',' . $g . ',' . $b;
	}
	else {
		// provide a default in an emergency
		return "128,128,128" ;
	}
}

Then in your theme:

$myrepeater = get_field('my_repeater');
foreach ($myrepeater as $item) {
    $hex = $item['color_subfield'];
    $rgb = sqcdy_get_acf_rgb($hex);
    // do stuff with $rgb
}