Use this green when letting a user know that a process has completed successfully or that all is well. #31A677 is 6.9:1 with black text.
Often blue is good for giving users update messages that don't require their input nor even for them to notice. #6CACC9 is 8.4:1 with black text.
On a white or near-white page, black or near-black text is generally a safe default color combination. #EEE is 18.1:1 with black text.
Yellow signals to users that something may require their input, dependent on factors (which you should convey). #DAD06F is 13.2:1 with black text.
Red is the common Western default for letting users know something has gone wrong. Avoid harsh reds. #BB6673 is 5.3:1 with black text, 4:1 with white.