If you see pink/purple tints on seedlings a few days after planting, or perhaps yellowing leaves, the chances are that your plants are not getting the nutrients and water that they need.
It may be that you start to see a purple edge developing around the leaves of peas, or quite commonly brassicas. Sometimes the leaves can turn completely purple. The purple is usually a sign of phosphorous deficiency, and this can happen even if the plants are growing in fresh compost – it can happen if the plants are too cold, or unable to draw up nutrients from the soil because they do not have enough water.
Squash and bean plants are more likely to display nutrient deficiency by turning yellow and blotchy, or patchy white/brown sometimes. Again, at this time of year, this can be related to temperature, but it can also suggest they are unable to access the nutrients in the soil.
What to do? Check if the soil is moist. If it is, then you can assume the plants are struggling with cold. In this instance, pop a cloche over the top of the plants, or cover with fleece until the temperatures warm to a more consistent level. They should bounce back fairly quickly once they are warm enough.
Of course, if the soil is dry, then water regularly and hopefully you will see them return to a lovely vibrant green within a few days. If growing without any fresh compost added, it may be worth giving a liquid feed to replace any nutrients in your soil.