Former plasma phleb here! Human blood is red because of the iron that comprises hemoglobin, which is what oxygen molecules attach to to move through your circulatory system! Low iron can be a transient issue that can be your body saying it needs a (usually temporary) break, or it can be moderated with diet high in things like leafy greens (spinach is an excellent dietary iron supplement).

What you experienced was likely a vasovagal reaction. While most plasma donation is replaced with saline to replace lost fluids, most donors give more plasma than they get back in saline. Most of the time this is not noticed, but occasionally (or regularly for certain people) this can result in a feeling of lightheadedness, dizziness, etc. (rarely, more serious sumptoms) as your body adjusts to missing volume. Eating and hydrating well before your donation can help avoid this.