Adding a symbol to a cell

Hi all,

I'm currently learning French and I'm making a verb conjugation table for the many different irregular forms French verbs can take.

I use colours to denote forms of a verb that follow the example of another verb's conjugation. recevoir for example follows pouvoir due to both having a short-form past participle.

While for now I denote irregular forms with a full red colour, I'm looking for another way to do this, because sometimes verbs have an irregular stem while still following another verb's pattern. Vivre for example has the irregular stem vécu while following the forms of vouloir

For this example one I used a red X in superscript but unfortunately I can't easily paste it over to other cells, not even with ctrl+f's replace function. It's quite a lot of work to create from scratch.

Is there a way to quickly add another symbol of sorts to a cell with which I can show this irregularity?