@NickD provided a good answer: use a face.
OP's comment to Nick's answer says that he'll try to write a function that, given a string, returns a propertized string. Such functions already exist: propertize does that, and so does add-face-text-property.
(setq ss (propertize "abcde" 'face '(:foreground "red")))
(setq ss &...
You need to create a string with the appropriate face. You do that by attaching a face text property to the string, giving it value of some face (predefined or defined for the specific purpose - you can look at all the predefined faces with M-x list-faces-display and pick one from there, or you can define your own face).
E.g. here's a snippet to use a ...