"Do I have a blue scarf?"
Translation:هَل عِنْدي وِشاح أَزْرَق؟
Azraq = masculine "blue"
Zarqá' = feminine "blue"
The same rhymes with the rest of colors.
أحمر (red/m) حمراء (red/f)
أصفر صفراء (yellow)
أخضر خضراء (green)
أبيض بيضاء (white)
أسود سوداء (black)
These are the main colors. Other colors that are derived from names (e.g. وردي - pink - which is derived from ورد - roses), such colors are changed to feminine by adding ta-marbúta ة only (وردي - وردية)
There is no single rule for specifying the gender of the noun in Arabic as it is in, Spanish, for example. However, I can say that if the noun ends with Ta-Marbúta ة there is a big chance that it is a feminine noun. Memory, I'd say, is your first aid in this matter (probably as it is the case with genders in German or Russian, but in Arabic it's simpler).