Learn Arabic (Modern Standard): السماء رمادية اليوم.

The word السماء (the sky) is an irregular feminine noun. Even though it lacks the typical feminine ending, it is treated as feminine in Arabic. Therefore, any adjective describing it must also take the feminine form, which is why we use رمادية instead of the masculine رمادي.

Where is the verb for 'is' in this sentence?

In Arabic, present tense sentences that describe what something is (called nominal sentences) do not use a verb for 'is' or 'are'. You simply place the subject (السماء) directly before the information about it (رمادية) to mean 'The sky is gray'.

The word for gray, رمادية, seems related to a noun. Where does it come from?

It comes from the noun رماد, which means 'ash'. By adding the adjective suffix (called a nisba), it literally translates to 'ash-colored' or gray. Many secondary colors in Arabic are formed this way by naming an object of that color.

Does اليوم mean 'today' or 'the day' in this context?

Literally it means 'the day', but when used as an adverb of time in a sentence like this, اليوم translates to 'today'. If you wanted to say 'the gray day', you would need to make 'gray' an adjective modifying 'day' directly, which would look like اليوم الرمادي.