Knowing when to replace bulbs

[smpross]

Hi All,
I have a hamilton MH fixture with (2) AB 14K 250 Watts. THey are about 10 months old. The tank lately has appeared a little more yellow than I remember it. Although it still looks as bright. The only change that I have made recently is putting the lights on new timers. So my question is, when lights need to be replaced, do they become more yellow in color? How do you know when it is time? How long should they last?

Thanks.

Scott