On the first day of my placebo week my period is usually dark red, but it was light red this time. Is this a problem?

Not a problem! It’s totally normal for the color and consistency of your period flow to change from day to day, month to month. As always, if you have concerns about pregnancy (like you missed a dose or two, or if you took medications off schedule) a pregnancy test is always advised, but I wouldn’t worry just because the color is a little different today/this month.