In Calc is there a way to change font colour for certain values?

For example if a cell number is lower than 70, it should be red, but if 70 or above black?

please go through the help file regarding format codes where in we can see the conditional formatting of the values please see the following link
we can also use the [BLUE][<70]#0 ;[RED][>70]#0 ;[BLACK]#0
in the format—> cells —> Numbers tab —>Numbers category —> format code

There is some documentation for this scenario. See also a previous Q&A which might also supply some relevant hints. (It’s a start, anyway!)