You are right is is uncountable (one would say a pice of chalk, two pieces of chalk, etc.) but in your example the plural "chalks" is a set term. So "coloured chalks" just implies there is chalk of many different colors in the box but does not imply a set number and does not work a counter.

This is a very interesting question. I think I agree with Richard that "box of chalks" is correct in British English but it is definitely not correct in American English. I looked at Google Images and you can see a few boxes that say "chalks" on the front, but there are many many more that say "chalk." My suggestion would be to say "box of chalk" since this is never wrong. If you are speaking to an American and say "box of chalks" they won't realize that this is actually correct in the UK and will assume you've made a mistake. You can always make "chalk" plural by saying X pieces of chalk or X sticks of chalk. Of course, this is only true when you mean 粉笔 -- chalk has other meanings and uses (e.g. chalk in powder form) that are never countable.

Bella - as you may know, there are differences between American and British English. Here in Canada we see both and use both. Older Canadians were taught to use the British system of spellings and grammar terms, e.g. colour (BR) vs color (US) and progressive tense (BR) vs continuous tense (US). Actually, I was surprised to read that "chalk" is countable, hence "chalks", however, the dictionary also stated that the noun chalk, when sepaking of chalk used for writing, is also uncountable. When uncountable, it must be used with a quantifier/determiner word, such as pieces of chalk. So, in conclusion, it appears that both usages are correct if you accept both American and British interpretations... interesting, yes? http://oald8.oxfordlearnersdictionaries.com/dictionary/chalk_1

No chalk is uncountable.