What does it mean if my vomit was yellowish?

Biliary emesis. Bile is typically a golden yellow color. It mixes with food to help digest dietary fats. When bile backs up into the stomach, it may appear in your vomitus. When persistent, it can lead to alkaline reflux and stomach irritation. Lots of common things can cause bilary emesis, but if persistent, worsening, or associated with pain, tenderness, weight loss, blood then please have it evaluated.
Not much. Appearance of vomitus can obviously vary. We are concerned if bilious (dark green) vomiting occurs and persists for some time. Bloody vomitus is also a concern, along with the classic "coffee grounds" appearance that can indicate bleeding in the upper GI tract. Beyond that, the color/appearance of vomitus is not all that concerning.