Green has higher luminance than blue at equivalent saturation. The values for SMS and iMessage background colors are, respectively and in sRGB, #00CC46 and #0080FF, corresponding to relative luminance values of 0.436 and 0.227 according to the WCAG 2 formula.
With white foreground text, this gives a contrast ratio of 2.15:1 for SMS and 3.79:1 for iMessage. WCAG 2.x AA level compliances requires a contrast ratio of at least 4.5:1 for normal text and at least 3:1 for large text.
https://www.w3.org/WAI/WCAG21/Understanding/contrast-minimum...