Lower Tomato Leaves Changing Colour

My Red Alert tomatoes are only about 6 inches tall and within an unheated greenhouse, watered from beneath.

It's the first time I've grown tomatoes and have noticed recently that many (but not all) of the lower leaves are discoloured however the rest remain healthy looking, is this common?

The "baby" leaves have done their job so that's why they go like that. No worries!

I agree with DD & GG but in addition:

You say you're watering from below. Are you standing the pot in water? If so are you drowning it (a bit)? At this time, I believe tomatoes should be kept damp, but not wet. The roots need to be encouraged to grow which overwatering will not and, (I learned this this year), if the plant cant get enough nutrient from the soil (perhaps because the roots are too wet) it will take nutrient from the lower leaves to feed the growth.

It's only my third year growing tomatoes, but to me the medium you have them growing in looks like it might be overly water retentive, which won't help as Headgardener suggested.

Mine are in 3 inch pots with a mix of compost, top soil, and perlite to add drainage. I haven't watered them since they were transplanted from cells to their pots 3 weeks ago, and they are also loosing a little colour in their lower leaves (in most cases), so even with added drainage (I used about 1/3 perlite), there might not be enough for them.

If they were my plants I'd be tempted to pick out the worst of the lot, tap them out of their pots, carefully remove some (no more than 1/2) of the medium around the roots, and re-pot with new mix containing a generous helping of perlite/grit/sand to replace the removed medium. If they are looking significantly better in a couple of weeks then you know what you need to do with the others.

If you are new to tom growing the next panic will be when the lower leaves start to go a mottled yellow. Get a packet of Magnesium Sulphate (Epsom salts) in ready!
Most tom plants when they really get growing tend to suffer from magnesium deficiency and it is an easy one to put right !

I water it down and put it on the roots  or even sprinkle the ES around the roots and water it in !
Have never done anything but good either method !