Others have already said this, but this needs clarification; it's okay to be honest about not liking something. It's okay to point out when something is legitimately terrible. An honest opinion on the quality of the work is what reviews are for.
However, there are ways to do this that are better than others. "I did not like this work at all, let me explain the reasons why" is a better way to do it than "oh my god this work sucks and I want to set it on fire". They're both negative, but one of them is certainly more useful than the other.
... I honestly find this to be a useful critique. It points out three specific problems that the creator can work on and it's not hostile at all. The creator now knows they should think about tightening up the pacing (i.e: spend less panels per scene), straighten up the drawings a bit and pay more attention to the lettering. It could do with some mentions of what the comic DOES do well, but all in all, it's not a terrible review. It's just not very long.
Whenever I review a comic, I tend to use the sandwiching technique mentioned in this thread - start out with positive first impressions, go on to the flaws I found and what I think can be done about them, and then end on a positive encouraging note, but if I received one of the one-sentence reviews like the example you posted above, I would find it honestly useful.
When you ask for a review (which is how most of us indie creators get in-depth reviews), you're already kind of implying to the reviewer that you're doing it BECAUSE you want to improve. If that urge to improve disappears because someone said "Sorry, your comic needs work." then you weren't really looking for an honest review - you were looking for praise. Those are two different things.
I do not in ANY way encourage harshness as a habit for reviewers; your review isn't better because you're harsh and unforgiving - but at the same time, when a creator asks for a review, they need to go in knowing that it might be negative (and do see my earlier statements on levels of negativity). It's always worth keeping in mind who the review is addressed to; never be dishonest and say you like something when you don't, but adjust the depth of the review depending on who is asking for it.
Addressing a review to someone who has a decade+ of drawing comics behind them is different from addressing a review to a 14 year old who has been drawing comics for a few weeks, you know?
Both of these are valid critiques, and I don't find the first one particularly harsh. It's just a statement of fact, and a perfectly neutral one. I would probably phrase it as "Your colouring needs some work. Here are the particular things that stand out of me: [examples.] From now on, keep [colouring method/fact/colour theory rule] in mind."