Is hiding invisible keywords in the background of a site penalised, and how can I explain that to a client?

I'm pretty new to making websites, I currently have 2 clients I'm just using wix and wordpress for. One of them has asked me if I can double up text they have written for their website or add keywords and make them the same colour as the background to improve SEO, I've told them that search engines might not like this and it could work against their SEO but they want me to do it anyway. Like I say I'm new to this, and I was a decent site to put on my portfolio at the end of this, has anyone got any idea how I might be able to better explain how this is bad for SEO. Alternatively, would this work if I kept it below a certain amount of words?