Would it be okay to use a volume 30 creme developer with dark brown hair dye?

no need for 30 volume when going darker, as a matter of fact i would use a semi-permanent if going darker. no need for permament color unless you are planning to stay dark for awhile, but if you like to change your hair color a couple times a year the semipermanents are easier to lift out of the hair than the permanents will be, especially if you are using box color, these usually contain metallic dyes which are also hard to remove if you want to lighten at any point..goodluck!