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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.

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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.

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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.

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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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.

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.

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.