Having said that, I am really confused on the contextual usages of bright and dark and I would be really grateful if someone clears this for me.

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Brighter - having or reflecting elevated luminance level. Lighter - having or reflecting elevated luminance level. Darker - having or reflecting reduced luminance level.

When talking about colourspaces, brightness general refers to things that reflect a large amount of light. It's also useful to refer to it as an HSL coordinate.

This filter can be applied as a non-destructive, live filter. It can be accessed via the Layer menu, from the New Live Filter Layer category.

High brightness so frequently results in high contrast elsewhere in life, there's a tendency to use the two terms interchangeably. However, in this particular instance, a darker (less bright) colour would provide better contrast and thus better visibility against a white background. If the presenter was writing on a black chalkboard in a dark (e.g. dark brown) colour, you would be right to ask for a "brighter" colour (e.g. bright yellow) to provide a greater contrast and hence visibility.

I recently attended a conference where the presenter was using a whiteboard marker to explain and most of the times the items he drew or wrote on the board were not visible due to the lack of ink in the marker. I just interrupted and asked him to use a bright colored marker so that the items are visible.

What you were really asking for was a colour that (all else being equal such as line thickness) had better contrast with the whiteboard. There a couple of issues regarding your colleague's correction.

A less confusing answer might have been to use a marker with a higher contrast with the whiteboard. In short, avoid making a request where terms introduce sources of error or incorrect interpretation. Allow the performer to evaluate the result of using the marker of their choice.