Hi Kwanhathai,

Sometimes colonies seem white, but they are actually very light blue. Are you sure this is not the case? Put the plate against white sheet of paper to be positive.

Dragana has a point. Sometimes the blue color can take quite a while to develop. Leaving your plates at room temperature overnight sometime helps. 

I suspect also that your colony density might be too high. Try doing a couple of tenfold dilutions. 

Check that your LB doesn't have added glucose. This would suppress lac expression via catabolite repression. 

In that you get blue colonies at the edge of your plate, I would guess your XGAL is ok. I'm not so sure about your IPTG. Try increasing the concentration of both. 

Spreading IPTG and XGAL into your plates is a quick and easy method, but for optimal results you should mix them into your agar. 

Spreading a lac+ E. coli on one of your plates would be a handy control. 

You can also try S-Gal (Sigma) which gives distinct black colonies. Mixing it with agar is always better than spreading it later on the plate.

Thank you everyone. 

I will try to increase the X-gal and IPTG concentration. 

If it doesn't work, I will try with S-Gal.  

In this RACE kit, the provided vector was linerlized and the clone tech  was called  Infusion, only when the linerlized vector was ligated with the insert can the clonies grow. So I think all the white clonies were right, check that please!