Text color chosen in post editor is overwritten

I’ve realised that, when writing a post, I can choose several colors for my text with a format button WordPress editor has. But then when the post is published, all the text shows in black. The colors are not applied, though if I look at the HTML, a span tag has been created wrapping the words that were colored, but with no style on it.

Are the theme styles overwritting the colors? Seems so but I can’t find how or where. Thanks for your support.